raymond f. gasser
ATLAS OF HUMAN EMBRYOSCopyright © 1975 RF Gasser, PhD. All rights reserved. Reproduced with permission.
Chapter VII (Pages 163-240)
- External Appearance
- Nervous System
- Alimentary and Respiratory Systems
- Urogenital System
- Coelomic Cavity
- Cardiovascular System
- Skeletal System
- Muscular System
- Integumentary System
- Special Sense Organs
the seventh week of life
cleft lip period
The specimens measure approximately 14 to 20 mm in length.
The head enlarges considerably and exhibits bulges produced by the cerebral vesicles. As the head raises from the ventral chest wall, the neck becomes better defined. A redundant layer of ectoderm called the cervical fold is sometimes visible in the cervicomandibular area.
The optic cup begins to migrate from the side of the head to the face. Upper and lower eyelid folds appear in the ectoderm above and below the cup, respectively.
The branchial arches and grooves disappear as the opercular fold covers over the more caudal arches. The first groove is surrounded by hillocks from which the auricle develops. The proximal part of the maxillary and mandibular processes blend with each other forming the cheek. The primitive rima oris is located between the distal part of the two processes. The surface features of the face are described in Section III, A.
The arm, forearm and hand become evident in the upper limb bud. The thigh, leg and foot appear in the lower limb bud. Five ridges separated by grooves form at the periphery of the primitive hand and foot plates and represent the digits. The limb buds rotate in such a way that what was formerly the ventral surface now faces medially. The ventral surface or palm of the hand is adjacent to the chest wall. The ventral surface or sole of the foot lies adjacent to the umbilical cord. Thenar and hypothenar areas of the palm are represented by elevations at the base of the thumb and little fingers, respectively.
The heart and liver form prominences in the ventral body wall.
A small bud on the lateral chest wall called the mammary gland primordium is the only remnant of the mammary ridge.
Although the umbilical cord remains very large, its size relative to the embryo is reduced.
The surface features of the external genitalia (perineal region) are described in Section IV, E.
All that remains of the tail bud is a short, blunt projection in the midline between the buttocks.
A. CENTRAL NERVOUS SYSTEM (CNS)Fig. 7–1B
Since the entire brain enlarges at a faster rate than the spinal cord, the latter becomes relatively smaller and shorter.
The primitive meninx differentiates into an outer, condensed layer called the ectomeninx and an inner, loosely arranged layer called the endomeninx located adjacent to the surface of the brain and spinal cord.
The striatal part enlarges considerably and bulges into the third ventricle at the level of the interventricular foramen where it produces the striatal ridge.
The primordial piriform cortex (paleopallium) differentiates at the surface of the striatal part and will receive secondary olfactory fibers.
The suprastriatal part expands rostrally, dorsally and caudally producing the pallium (primordial cerebral cortex). The caudal portion begins to cover over the diencephalon.
The layer of tissue in the midline that separates the pallium on each side will become the falx cerebri. The medial wall of the pallium thins and invaginates into the lateral ventricle to form the tela choroidea. The line of imagination, called the choroid fissure, first appears at the level of the interventricular foramen.
Dorsal to the choroid fissure the medial wall bulges into the lateral ventricle as the primordial hippocampus. The clear outer layer represents the primordial hippocampal cortex (arch/pallium). It can be subdivided into a ventral area, the primordial gyrus dentatus, and a more dorsal area, the primordial cornu Ammonis. The medial surface in front of the lamina terminalis is the precommissural septum (parolfactory area). A small prominence on the ventromedial surface represents the primordial olfactory bulb, which receives the olfactory nerve fascicles. Caudal to the bulb is the primordial tuberculum olfactorium.
Most of the pallium lies between the primordial hippocampal cortex and primordial piriform cortex. This large area, referred to as the primordial neopallial cortex, is associated with non-olfactory sensations and will form many connections with the thalamus.
The preoptic area and recess are seen rostral to the optic chiasma of the diencephalon in the floor of the third ventricle.
The lamina terminalis in front of the recess represents the rostral end of the brain. It gradually diminishes in relative size and is covered by the pallium.
The paraphysis forms as an evagination of the paraphyseal arch in the roof of the third ventricle near the tel- and diencephalon junction. Usually it disappears during early fetal development (100 mm).
A third sulcus, the sulcus medius, appears in the lateral wall between the sulcus dorsalis and sulcus ventralis. The three sulci divide the wall into four areas. From dorsal to ventral they are the epithalamus, dorsal thalamus, ventral thalamus and hypothalamus. A lightly staining, less cellular area called the zona limitans intrathalamica becomes evident near the sulcus medius and helps to separate the dorsal thalamus from the ventral thalamus. Increased proliferation in the mantle layer causes the hypothalamus to bulge into the third ventricle.
The optic chiasma becomes a thick band in the rostral part of the floor plate that connects the optic stalks across the midline. Caudal to the chiasma the solid neurohyphyseal bud is flanked on each side by the tuberal part of the adenohypophysis. The mamillary area remains as a slight outward bulge in the caudal part of the floor plate.
No significant changes occur in the pineal bud area in the caudal part of the roof plate.
Lamination begins to appear in the mantle layer of the tectum.
The tegmentum and basis pedunculi area subdivisions increase in size. A midline depression called the inter-peduncular fossa forms between the peduncular area on each side in the region of the cephalic flexure.
As the mesocoele narrows it becomes the cerebral aqueduct connecting the third and fourth ventricles.
Cranial nerve III attaches to the wall of the interpeduncular fossa. Cranial nerve IV attaches to the roof of the isthmus.
FIG. 7 –1
FIG. 7 –1 [printable version of FIG 7-1]
- External features of the 18-mm embryo.
- Central nervous system of the 18-mm embryo. Arrows indicate the position of the cephalic, pontine and cervical flexures.
The pontine flexure becomes more acute.
The thin, diamond shaped roof of the fourth ventricle continues to enlarge with the cranial part forming the roof of the metencephalon and the caudal part forming the roof of the myelencephalon.
There is no longer any evidence of the rhombomeres.
A midline raphe of crisscrossing fibers becomes prominent in the floor plate region along the entire length of the rhombencephalon.
As the marginal layer of the basal plate thickens around the attachment of the trigeminal nerve it becomes the basal portion of the pons. The ventral median sulcus becomes apparent on the ventral surface.
The remainder of the basal plate together with the medial segment of the alar plate constitute the tegmentum of the pons. Neuroblasts in the mantle layer of the basal plate will form the motor nuclei of cranial nerves V, VI and VII. Neuroblasts in the alar plate will contribute to the formation of the sensory nuclei of cranial nerves V, VII, VIII and X.
Cranial nerve V attaches to the metencephalon in the middle of the pons area. Cranial nerves VI, VII and VIII attach at the caudal edge of the pons area.
Rhombic Lip (Cerebellum)
The rhombic lip is a slight thickening of the lateral segment of the alar plate where the roof of the fourth ventricle attaches. It can be divided into intraventricular and extraventricular portions.
The myelencephalon becomes a distinct subdivision of the adult brain between the pons area and the C-1 spinal nerve called the medulla oblongata.
Neuroblasts in the basal plate will form the motor nuclei of cranial nerves IX, X, XI (cranial part) and XII. Neuroblasts in the alar plate will contribute to the formation of the sensory nuclei of cranial nerves V, VII, VIII, IX and X.
The pyramidal tract area thickens on each side of the midline raphe.
The rootlets of cranial nerve XII attach just lateral to the pyramidal tract area. Cranial nerves IX, X and XI attach in series to the lateral wall of the myelencephalon.
The spinal cord does not keep pace with the enlargement of the brain and becomes relatively narrower and shorter.
The layers increase in thickness converting the neural canal into a vertical slit that is slightly expanded at the sulcus limitans.
Caudal to the coccygeal spinal ganglion the spinal cord narrows and ends abruptly. It continues into the tail bud remnant as the primitive filum terminale. The primitive filum terminale is initially very short and contains a tiny canal that is continuous with the neural canal.
B. PERIPHERAL NERVOUS SYSTEMFig. 7–2A
The diameter of the cranial and spinal nerves together with their sensory ganglia is relatively smaller since the nerves can not keep pace with the enlargement of the CMS.
Many named branches become apparent as the nerves course into the peripheral areas.
Olfactory nerve (I)—The olfactory nerve passes as a group of fascicles from the olfactory epithelium to the primordial olfactory bulb. The vomeronasal nerve and nervus terminalis course from the vomeronasal organ and front part of the nasal septum, respectively, to the medial side of the primordial bulb.
Optic nerve (II)—The optic stalk lumen is barely visible because of the increased quantity of fibers from the optic cup causing the lumen to narrow. After the lumen disappears, the optic stalk is called the optic nerve.
Oculomotor (III), trochlear (IV) and abducens (VI) nerves— These nerves terminate in distinct premuscle masses surrounding the optic cup.
Trigeminal nerve (V)—The three divisions or branches of the trigeminal nerve course into their particular area of the face. The infraorbital branch of the maxillary nerve and the inferior alveolar and lingual branches of the mandibular nerve become apparent.
Facial nerve (VII)—The facial nerve does not yet spread onto the face but instead terminates in the cervicomandibular area. More proximally the chorda tympani, greater petrosal, posterior auricular and digastric branches can be identified. The chorda tympani branch joins the lingual nerve.
FIG. 7 –2
FIG. 7 –2 [printable version of FIG 7-2]
- Peripheral nervous system of the 18-mm embryo. In most instances, only the most proximal portion of the nerve is shown.
- Alimentary and respiratory systems of the 18-mm embryo.
Vestibulocochlear nerve (VIII)—Short branches from the vestibular ganglion course to the future receptor area of the semicircular canals, saccule and utricle. The cochlear (spiral) ganglion gives branches to the cochlear pouch and spirals along with it.
Glossopharyngeal nerve (IX)—Before it terminates in the surface on the caudal part of the tongue, the glossopharyngeal nerve gives a minute branch to the stylopharyngeus premuscle mass.
Vagus nerve (X)—The vagus nerve joins its counterpart on the other side around the esophagus to form the esophageal plexus, which passes into the upper abdominal viscera as anterior and posterior vagal trunks.
Spinal accessory nerve (XI)—As the sternocleidomastoid and trapezius muscles become separate masses, they each receive a separate branch from the spinal accessory nerve.
Hypoglossal nerve (XII)—Separate branches of the hypoglossal nerve course to the muscles of the tongue as each muscle differentiates.
The branches of the nerve plexuses can be identified.
C. AUTONOMIC NERVOUS SYSTEMFig. 7–2A
Neuroblasts giving rise to postganglionic neurons collect together to form identifiable ganglia near their definitive position.
SYMPATHETIC NERVOUS SYSTEM
The individual ganglia of the sympathetic trunk become apparent. The superior, middle and inferior cervical ganglia are especially prominent because of their large size.
Many sympathetic neuroblasts collect together in the vicinity of the celiac, superior mesenteric and inferior mesenteric arteries to form preaortic ganglia with the same names. These ganglia are connected to the lower thoracic segments of the spinal cord by the splanchnic nerves and rami communicans.
Suprarenal gland—The cortex is a large acidophilic mass between the dorsal aorta and the cranial end of the mesonephros and gonad. The medulla is dorsomedial to the cortex and is composed of prechromaffin cells.
PARASYMPATHETIC NERVOUS SYSTEM
Parasympathetic neuroblasts collect in the head region to form four ganglia in close association with the divisions of the trigeminal nerve. Their origin is not clear as they are influenced by cranial nerves other than the trigeminal. The ganglia are as follows:
The ciliary ganglion lies close to the ophthalmic division near the optic cup but receives its messages from the brain by the oculomotor nerve.
The pterygopalatine ganglion is located on the maxillary division but receives brain messages by way of the facial nerve.
The submandibular ganglion is close to the mandibular division but is also connected to the brain by way of the facial nerve.
The otic ganglion is located on the mandibular division but receives impulses from the brain through the glossopharyngeal nerve.
The parasympathetic neuroblasts associated with the general visceral branches of the vagus nerve are scattered throughout the thoracic and upper abdominal viscera. Their origin is unclear but they may arise from crest cells in the rhombencephalic region. Since these neuroblasts never collect in large numbers, ganglia are not described.
Neuroblasts collect in the vicinity of the pelvic viscera and receive messages from the sacral part of the spinal cord by way of the pelvic splanchnic nerves. They also have an unclear origin but likely are derived from the neural crests in the sacral region.
A. ORONASOFACIAL REGION
The nasal pit on each side of the midline deepens to form the primitive nasal cavity. The cavity opens to the exterior at the external nostril and is separated from its counterpart on the opposite side by the nasal septum. The olfactory epithelium lies in the roof of the cavity. A small epithelial invagination called the vomeronasal organ forms in the medial wall of the cavity on each side of the septum.
The stomodeum is replaced by the primitive oral cavity. The primitive nasal and oral cavities are initially separated by a thick oronasal membrane. The membrane soon thins and ruptures thereby communicating the two cavities through an opening called the primitive choana. The primitive choana enlarges in the roof of the primitive oral cavity on each side of the nasal septum. Rostral to the primitive choana the nasal septum joins an area of the primitive oral cavity roof called the primitive palate. The primitive palate is continuous with the medial nasal and frontonasal elevations. Because it lies between the maxillary processes, it is also called the intermaxillary segment. Remnants of the nasal fin are sometimes apparent between the primitive palate and maxillary process. As the lateral wall of the primitive oral cavity enlarges, the inner side of the maxillary process protrudes into the cavity forming the lateral palatine process. The rostral part of the lateral palatine process is in contact with the lateral surface of the tongue. Its caudal part lies below the tongue in the linguogingival groove where it remains until shortly before palate closure.
The nose begins to appear around the external nostrils when the frontonasal elevation becomes prominent and blends with the nasal elevations. The maxillonasal groove disappears after its walls fuse to produce the nasolacrimal duct. The duct courses from the medial aspect of the eye and terminates in the primitive nasal cavity.
The maxillary process blends with the nasal elevations. The mandibular process joins in the midline with its counterpart on the other side causing the median mandibular groove to disappear. The dorsal adjacent portions of the mandibular and maxillary processes blend to produce the cheek. Their ventral portions contribute to the formation of the primitive rima oris that replaces the stomodeal opening.
The roof and lateral walls of the primitive oral cavity grow rapidly to keep pace with the expansion of the rostral part of the brain. As a result the point of attachment of the hypophyseal pouch to the roof is shifted caudally to a position dorsal to the foregut floor. The junction of the pouch with the roof narrows to produce the solid stem remnant. Dorsal to the remnant the pouch retains its lumen and forms the adenohypophysis. The adenohypophysis becomes U shaped flanking each side of the neurohypophyseal bud. The lower connecting part of the U becomes the distal portion, the upper part on each side is the tuberal (infundibular) portion.
The epithelial lining of the primitive oral cavity in the vicinity of the maxillary and mandibular processes thickens in a U shaped area to form the labiodental lamina. The lamina separates into an inner dental lamina and an outer labiogingival band.
The parotid and submandibular buds appear as epithelial sprouts of the primitive oral cavity lining. The parotid bud begins in the cheek; the submandibular bud develops in the floor of the linguogingival groove.
The tubotympanic recess (pouch 1) elongates in a dorsolateral direction to form the auditory tube and the primitive tympanic cavity. The second pouch reduces to the shallow palatine fossa. The third and fourth pouches lose their connections with the primitive pharyngeal wall. Their derivatives migrate into the ventral aspect of the neck. The dorsal portion of the third pouch becomes the caudal parathyroid bud, its ventral portion becomes the greater thymic bud. The two portions remain connected for a short period. The dorsal portion of the fourth pouch becomes the rostral parathyroid bud. The fate of its ventral portion is unclear but it probably contributes to the formation of the thymus (lesser thymic bud). The ultimobranchial body (pouch 5) is probably incorporated into the thyroid gland.
The thyroid gland loses its connection with the floor of the primitive pharynx and develops right and left lobes that are joined across the midline by the isthmus.
The swellings in the floor of the primitive pharynx blend to produce a massive tongue, which bulges into the primitive oral as well as pharyngeal cavities. The rostral part of the tongue becomes round and narrows to a tip near the primitive choanae.
The pharyngeal constrictor muscles become evident in the condensation peripheral to the endodermal lining. Loose mesenchymal tissue between the primitive pharynx and the vertebral column forms the retropharyngeal space. The caudal part of the primitive pharynx that passes dorsal to the larynx is called the laryngeal pharynx.
Caudal to the larynx the pharynx narrows to form the esophagus. The muscularis layer is apparent in the condensation surrounding the endodermal tube. Peripheral to the muscularis layer the vagus nerves communicate to form the esophageal plexus, which regroups at the caudal end of the esophagus to form the anterior (ventral) and posterior (dorsal) vagal trunks.
Stomach rotation is almost complete. The pyloric antrum is the slightly dilated portion to the left of the pylorus. The four layers of the stomach, mucosa (endodermal tube), submucosa, muscularis and serosa, become evident.
The boundaries of the spleen become discrete in the dorsal mesogastrium as it moves to a position dorsolateral to the stomach. The splenic blood vessels course through the dorsal mesogastrium to the spleen, stomach and dorsal pancreas.
The dorsal pancreas is represented by a branching duct that empties into the cranial part of the duodenum.
Laryngeal cartilages begin to appear in the condensation around the laryngeal orifice. The orifice narrows to a vertical slit called the primitive glottis.
Tracheal cartilages begin to form in the condensation around the endodermal tube. The tracheal bifurcation migrates to the level of the upper thoracic segments. The secondary bronchi divided into numerous tertiary branches called bronchopulmonary buds near the periphery of the lung sac.
The lung sac begins to assume its definitive shape with a base at its caudal end and an apex at its cranial end. An oblique fissure in the left lung sac divides it into upper and lower lobes. In addition to the oblique fissure the right lung sac has a horizontal fissure. These two fissures divide the right lung sac into upper, middle and lower lobes.
The liver increases tremendously in size enlarging more on the right than on the left. As the liver expands, the gall bladder enlarges and moves away from the bile duct. The proximal part of the gall bladder does not enlarge but becomes instead the cystic duct that empties the bladder into the bile duct.
The ventral pancreas moves to the left by passing dorsal to the duodenum. It is separated from the dorsal pancreas by the portal and superior mesenteric veins.
The duodenum assumes the shape of an arc coursing first dorsally and to the right, then caudally and to the left. It becomes continuous with the cranial limb of the midgut on the right side of the dorsal mesentery and superior mesenteric vessels. The lumen of the duodenum is partly obliterated in some areas but will become recanalized.
The midgut loop elongates at a rapid rate causing a greater portion to reside in the umbilical cord. It continues to rotate in a counterclockwise manner with the caudal limb passing to the left, above the cranial limb.
The primitive cecum lies in the proximal part of the umbilical cord and becomes the cecum after a diverticulum representing the appendix develops.
The muscularis of the midgut begins to differentiate from the mesenchymal condensation around the endodermal tube. It forms in the cranial limb before the caudal limb.
The hindgut endodermal tube is surrounded by a thick condensation from which the muscularis differentiates. Differentiation of the muscularis in the hindgut lags behind that in the foregut and midgut.
When the urorectal septum joins the cloacal membrane, it divides the membrane into a ventral urogenital part and a dorsal anal part. The rectum terminates at the anal part, the urogenital sinus terminates at the urogenital part. See Section IV for discussion of the development of the urogenital sinus.
The area on each side of the anal membrane swells forming anal folds. The ectodermal depression between the folds is called the proctodeum.
The number of segments occupied by the mesonephros is reduced, extending approximately from T-9 to L-3. The corpuscles and tubules in the cranial segment begin to degenerate.
The mesonephric duct is narrow at its cranial end but enlarges as it courses caudally on the ventral aspect of the mesonephros to terminate in the urogenital sinus.
The female (paramesonephric or Müllerian) duct, which will become the main genital duct in the female, begins as an invagination of the coelomic epithelium on the ventrolateral surface of the urogenital ridge. Initially the invagination is a solid bud which later canalizes. The duct grows caudomedially toward the pelvic part of the urogenital sinus. As it approaches the sinus, it meets and fuses with its counterpart on the other side.
FIG. 7 –3
Urogenital system of the 18-mm embryo. [printable version of FIG 7-3]
B. METANEPHROS (KIDNEY)
The primitive renal pelvis bifurcates into cranial and caudal major calyces within the metanephrogenic mass. Each major calyx branches deeper into the mass thereby forming collecting tubules. The distal end of each tubule is covered by a condensation called the renal capsule that is derived from the cells of the metanephrogenic mass.
The metanephros attains its definitive position ascending to the level of the T-12 to L-2 segments.
The proximal segment of the metanephric diverticulum becomes the ureter. As the horn of the urogenital sinus is absorbed into the main chamber, the entrance of the ureter is relocated lateral to the entrance of the mesonephric duct.
C. UROGENITAL SINUS
The urorectal septum completely separates the primitive urogenital sinus from the more dorsal rectum. The resulting definitive urogenital sinus is divided into three portions: a cranial, dilated bladder portion, an intermediate, narrow pelvic portion and a caudal phallic portion. The phallic portion is essentially a vertical slit between the urogenital folds and is separated from the exterior by the urogenital part of the cloacal membrane.
The allantois is continuous with the apex of the bladder portion of the urogenital sinus. As the lumen in the junction area disappears, the allantois is replaced by a solid, ligamentous structure called the urachus or median umbilical ligament.
D. GONAD (INDIFFERENT STAGE)
The gonad in the indifferent stage is a lobular mass extending from approximately T-10 to L-2 segments. It attaches to the medial side of the mesonephros by the gonadal mesentery where it comes into contact with some of the mesonephric tubules.
The primordial germ cell from the yolk sac reside in the gonadal ridge. As they enter the gonad, the surface cells of the ridge proliferate into gonadal cords that extend into the underlying mesenchyme around the germ cells.
E. EXTERNAL GENITALIA
Since the external genitalia is identical in the male and female, it is not yet possible to distinguish the sex from the surface features.
The cloacal membrane remains in the midline and is subdivided into urogenital and anal parts. The urogenital part separates the phallic part of the urogenital sinus from the urogenital groove. The anal part separates the rectum from the proctodeum.
The urogenital groove is flanked on each side by a prominent urogenital fold. The proctodeum is flanked on each side by an anal fold.
The genital tubercle in the midline and the more lateral labioscrotal swelling become prominent raised areas.
The originally common coelomic cavity is partitioned into three separate cavities: the peritoneal cavity surrounding the abdominal viscera, the pericardial cavity surrounding the heart and a pleural cavity surrounding each lung. The pleuropericardial opening disappears completely, and the pleuroperitoneal opening is reduced to a minute communication dorsolateral to the liver.
Each cavity keeps pace with the related expanding viscera and is completely lined with a serous membrane that is given different names in each cavity (peritoneal cavity → peritoneum; pericardial cavity → serous pericardium; pleural cavity → pleura). The part of the serous membrane that covers the inner surface of the wall is called the parietal layer; the part that covers the surface of the organ is called the visceral layer. The visceral layer is also called the serosa on the abdominal organs. On the heart it is included in the epicardium.
The serous membrane produces serous fluid that completely fills each cavity. This fluid serves as a lubricant that allows for easy movement between various viscera or between viscera and the wall that encloses them.
A. DORSAL MESENTERY
The segment of the dorsal mesogastrium between its midline attachment and the spleen lies adjacent to the ventral surface of the left gonad, mesonephros, metanephros and suprarenal gland. It contains the dorsal pancreas and splenic vessels. The segment of the dorsal mesogastrium between the spleen and greater curvature increases in length to form a redundant fold called the greater omentum. The lesser sac that is enclosed by the dorsal mesogastrium enlarges as the greater omentum extends caudally. The pneumoenteric recess disappears.
Differential growth patterns in the stomach and duodenum bring the dorsal and ventral pancreas together in the mesoduodenum.
As the midgut loop elongates, its dorsal mesentery assumes a fan shape and is twisted in a counterclockwise direction.
The dorsal mesentery of the hindgut is pushed to the left by the midgut loop.
B. VENTRAL MESENTERY
No significant changes will occur in the lesser omentum and falciform ligament.
The diaphragm is a partition that separates the peritoneal cavity from the pericardial and pleural cavities. At this time it is largely composed of the septum transversum.
The pleuroperitoneal membranes expand to close the pleuroperitoneal opening and contribute to the dorsolateral part of the diaphragm.
Muscle tissue begins to differentiate in the dorsal part of the septum transversum where the phrenic nerves enter.
A. HEARTFigs. 7–4, 7–5
FORMATION OF THE SEMILUNAR VALVES
Three tubercles form on the inside of both the ascending aorta and pulmonary artery at the junction of the truncus arteriosus and conus cordis (truncoconal junction). As they grow into the lumen, they become cuplike and together form a semilunar valve in each vessel.
CLOSURE OF THE INTERVENTRICULAR FORAMEN
The dorsal (inferior) endocardial cushion expands into the interventricular foramen and fuses with the upper edge of the muscular portion of the interventricular septum. Simultaneously, it also fuses with the lower edge of the conus septum. As a result a thick partition is formed that closes the foramen. The partition later becomes thin and fibrous to form the membranous part of the interventricular septum.
FIG. 7 –4
Heart of the 18-mm embryo. (A-V, atrioventricular.) [printable version of FIG 7-4]
FORMATION OF THE ATRIOVENTRICULAR (A-V) VALVES
The tissue around each atrioventricular canal bulges into the canal and the ventricle below it. The bulges become flaps of tissue that are attached in some areas to the ventricular wall. Three flaps develop around the right canal forming the right atrioventricular (tricuspid) valve. Two flaps develop around the left canal forming the left atrioventricular (mitral) valve. The attachments to the ventricular wall are replaced by dense connective tissue cords called chordae tendineae, which are connected to fingerlike processes of ventricular muscle known as papillary muscles.
SEPTUM FORMATION IN THE RIGHT ATRIUM
The ostium primum disappears as the caudal edge of the septum primum fuses with the endocardial cushions.
Simultaneously the cranial part of the septum primum breaks down forming the ostium secundum.
Most of the septum spurium and the left venous valve become incorporated into the septum secundum obliterating the septovalvular space. This causes the septum secundum to lengthen along the right side of the septum primum partially covering the ostium secundum.
The right venous valve (valve of sinus venosus) grows caudally to join the fused endocardial cushion near the orifices of the inferior vena cava and coronary sinus.
CHANGES IN THE EXTERNAL CONFIGURATION
The heart moves into the upper thoracic region.
The left ventricle becomes conical shaped with its blunted end forming the apex of the heart.
The transverse part of the sinus venosus and the left sinus horn become incorporated into the coronary sinus in the dorsal part of the atrioventricular sulcus.
Venous channels from the right and left lung sacs join near the heart to form the common pulmonary vein, which empties into the left atrium.
B. ARTERIESFig. 7–5A
DERIVATIVES OF THE AORTIC SAC, AORTIC ARCHES AND PAIRED DORSAL AORTAS
After the heart moves caudally through the cervical segments, the neck region becomes apparent. The arteries between the heart and the brain necessarily increase in length and narrow in relation to surrounding structures. Many dramatic changes occur in the aortic sac, aortic arches and the paired dorsal aortas. In most instances the changes differ on each side.
Aortic sac—The segment of the left horn of the aortic sac between the origins of the fourth and sixth aortic arches becomes the proximal part of the definitive aortic arch. The corresponding segment of the right horn forms the brachiocephalic artery.
Aortic arches—On both sides, the common carotid artery is derived by an elongation of that segment of the horn of the aortic sac between the origins of the third and fourth aortic arches. The proximal segment of the internal carotid artery is formed by the third aortic arch, its distal segment by the cranial extension of the paired dorsal aorta rostral to the carotid duct. The external carotid artery, a new vessel, joins the internal carotid artery at its point of origin. On the left the distal part of the definitive aortic arch is formed by the fourth aortic arch and the segment of the paired dorsal aorta between the carotid duct and the seventh dorsal intersegmental artery; on the right side these segments form the proximal part (base) of the right subclavian artery. The proximal part of the sixth arch on each side becomes the stem of the right or left pulmonary artery. Its distal part on the right loses its connection with the dorsal aorta and disappears. The connection with the dorsal aorta is retained on the left forming the ductus arteriosus. The pulmonary arteries course to their respective lung through secondary channels located ventral to the primary bronchi.
Paired dorsal aortas—On both sides the segment of the dorsal aorta between the third and fourth arches (carotid duct) disappears. On the right side the segment between the origin of the seventh dorsal intersegmental artery and the junction with the left dorsal aorta also disappears. When this happens, the caudal segment of the left dorsal aorta becomes the stem of the descending aorta, and all of the blood for the lower half of the body flows toward the left.
As the cerebellum becomes established, the stems of the definitive arteries supplying it can be identified. The met- and myelencephalic arteries are replaced by the pontine, superior cerebellar and anterior and posterior inferior cerebellar arteries.
The seventh dorsal intersegmental artery forms all of the left subclavian artery and all but the proximal part of the right subclavian artery. The axial artery (definitive axillary artery) that passes through the brachial plexus to supply the derivatives of the upper limb bud is a lateral branch of its ventral division. The vertebral artery and thyrocervical trunk represent, respectively, post- and precostal anastomoses of its dorsal and ventral divisions with corresponding segments of more cranial intersegmented arteries. The costocervical trunk and internal thoracic (mammary) artery are formed, respectively, by precostal and ventral anastomoses of its ventral division with corresponding segments of more caudally situated dorsal intersegmental arteries.
The formation of the ascending aorta is described in Chapter 6, Section VII, A. The development of the aortic arch is described above. The fused portion of the dorsal aortas forms the descending aorta. Additional definitive arteries that can be identified are as follows.
The dorsal intersegmental arteries arising from the thoracic part of the descending aorta become the intercostal and subcostal arteries. Those from the lumbar part become the lumbar arteries.
The lateral segmental or mesonephric arteries supply not only the mesonephros but also the adjacent gonad and suprarenal gland.
Many of the definitive branches of the celiac, superior mesenteric and inferior mesenteric arteries (ventral segmental arteries) can be identified.
C. VEINSFig. 7–5B
The distal segment of the vitelline veins disappears at varying times. It sometimes remains prominent after the yolk stalk disappears, in which case it courses from the yolk sac through the umbilical coelom to the dorsal mesentery of the midgut loop where it joins the superior mesenteric vein.
When the duodenum changes to the shape of an arc, the superior mesenteric vein then passes ventral to the lower part of the arc, becoming the portal vein between the dorsal and ventral pancreas after it receives the splenic vein. The portal vein courses to the ductus venosus in the liver by passing dorsal to the upper part of the duodenal arc.
Cardiovascular system of the 18-mm embryo. [printable version of FIG 7-5]
The single umbilical vein enlarges and passes through the liver as the ductus venosus, which terminates in the inferior vena cava.
Afferent veins to the hepatic sinusoids arise from the ductus venosus close to the entrance of the portal vein. Efferent veins from the hepatic sinusoids drain into the junction where the ductus venosus empties into the inferior vena cava.
Left Common Cardinal Vein
The left common cardinal vein continues to narrow and eventually disappears so that it no longer drains into the coronary sinus.
Precardinal Vein Tributaries
Venous channels peripheral to the brain separate into superficial and deep portions. The superficial channels develop into sinuses within the ectomeninx. The deep channels become the cerebral veins. A narrow channel called the primitive transverse sinus passes ventrolateral to the pons area and connects the anterior and middle cerebral plexuses. A more prominent anastomosis is the primitive sigmoid sinus, which lies ventrolateral to the myelencephalon and connects the middle and posterior cerebral plexuses.
The stem of the anterior cerebral plexus receives the primitive tentorial sinus from the cerebral vesicle and the primitive marginal sinus from the diencephalon.
The rostral segment of the primary head vein between the internal carotid artery and trigeminal ganglion becomes the primitive cavernous sinus, which receives the primitive maxillary and supraorbital veins. The primary head vein becomes the precardinal vein lateral to the parachordal cartilage in the vicinity of cranial nerves IX, X and XI. In the cervical segments the precardinal vein becomes the internal jugular vein.
The internal jugular vein receives a large tributary in the cervicomandibular area that represents the external jugular vein. In the caudal part of the neck it is joined by the subclavian vein. The vessel between this junction and the heart is formed by the caudal segment of the precardinal vein and the common cardinal vein. On the right side the segment becomes the right brachiocephalic vein and superior vena cava. It disappears on the left when an anastomotic channel that will become the left brachiocephalic vein develops ventral to the aortic arch and connects the left internal jugular and subclavian veins with the superior vena cava.
The postcardinal vein either disappears or is incorporated into other channels. Its cranial segment on the right connects the azygos line vein that develops medial to the sympathetic trunk to the caudal segment of the precardinal vein. Its intermediate segment becomes a plexus dorsal to the kidney before it disappears completely. The interpostcardinal anastomosis between the caudal segments contributes to the formation of the distal part of the inferior vena cava.
The intersubcardinal anastomosis will become the major portion of the left renal vein. The cranial segment of the subcardinal vein on the right side dilates to form the suprarenal part of the inferior vena cava. On the left this segment becomes the left suprarenal vein. The caudal segment of the subcardinal vein on each side is incorporated into the gonadal vein.
A third longitudinal channel develops dorsomedial to the postcardinal vein and connects to both the postcardinal and subcardinal veins of the corresponding side. Only part of the caudal segment remains on the right side. It connects the postcardinal anastomosis with the intersubcardinal anastomosis forming the infrarenal part of the inferior vena cava. The sub- and supracardinal anastomosis on the left is incorporated into the renal vein.
Inferior Vena Cava
The inferior vena cava is the largest definitive vein in the body. It begins in the lower lumbar region at the junction of the common iliac veins and ends in the right atrium. It is a composite structure forming only on the right side from the dilated segments of four major venous channels. Beginning caudally these channels are the 1) interpostcardinal anastomosis, 2) right supracardinal vein, 3) right subcardinal vein and 4) right hepatocardiac vein (proximal right vitelline vein).
Distinct right and left pulmonary veins from their respective lung sac join near the left atrium to form the common pulmonary vein. The common pulmonary vein disappears as it becomes incorporated into the left atrial wall.
The jugular lymph sac enlarges and extends cranially to the upper cervical segments lateral to the internal jugular vein. Near its termination in the internal jugular vein it receives the axillary lymph sac from the proximal part of the upper limb.
An iliac lymph sac appears around the iliac blood vessels from the lower limb.
The mesenteric lymph sac begins as small spaces dorsal to the inferior vena cava and aorta.
Cartilaginous Neurocranium (Chondrocranium)
The parachordal condensation and occipital sclerotomes form the parachordal (basal) cartilage located between the hypophysis and the C-1 segment.
The mesenchymal condensation lateral to the hypophysis becomes the hypophyseal (polar) cartilage.
A narrow midline condensation in the septal region between the primitive nasal cavities represents the trabecular cartilage. It extends from the hypophysis to the nasal capsule condensation.
Cartilage appears in the otic capsule surrounding the semi-circular canals and cochlear duct. The otic capsule joins with the parachordal cartilage.
A condensation called the alisphenoid develops between the parachordal cartilage and the maxillary and mandibular nerves. It is separated from the otic capsule by the internal carotid artery.
The orbitosphenoid is a well-defined cartilage in the ectomeninx around the optic stalk. It is separated from the alisphenoid by the cranial nerves III, IV and VI.
The nasal capsule surrounds the primitive nasal cavity and unites with the trabecular cartilage.
The dorsal and lateral surfaces of the brain are covered by the ectomeninx, which will give rise to the flat bones of the skull.
The first and second branchial arch condensations transform into prominent bars of cartilage.
The third arch cartilage separates from the laryngeal condensation becoming a curved, fingerlike condensation at the root of the tongue. It joins the ventral end of the second arch cartilage where the two together will form the hyoid bone.
The remainder of the arch cartilages are represented as laryngeal condensations.
B. POSTCRANIAL PART OF AXIAL SKELETON
The caudal condensed part of each sclerotome forms the intervertebral disc.
The cranial, less-condensed part gives rise to the cartilaginous centrum. A cartilaginous neural arch projects dorsally from each side of the centrum. Each pair of arches is joined dorsal to the spinal cord by the ectomeninx. Projecting laterally from each centrum is a short transverse process.
RIBS AND STERNUM
The 12 pairs of rib primordia become cartilaginous and extend ventrolaterally into the body wall. The enlarged end of the rib adjacent to the centrum forms the head. The lateral, thin part is the shaft. A tubercle is present where the rib makes contact with the transverse process.
On each side the distal ends of the primordial ribs are joined together by a longitudinally arranged strip of cartilage called the sternal bar. The sternal bars are separated by the large heart and liver. Cranially the two bars join together in the midline to form the episternal cartilage.
C. APPENDICULAR SKELETON
There is a proximodistal sequence of differentiation in the limb buds with the pectoral girdle being slightly more advanced than the pelvic girdle. The skeletal components of both girdles with the exception of the clavicle becomes discrete pieces of cartilages. The clavicle is a dense membrane that begins ossifying very early, even before chondrification begins in other areas.
A. HEAD MESODERM
AROUND THE OPTIC CUP
The individual extraocular muscles become evident and receive an innervation from cranial nerves III, IV or VI.
Arch 1—The muscles of mastication are a large premuscle mass that receives fibers from the mandibular nerve. The premuscle mass is located lateral to the first arch cartilage between the optic cup and the first branchial groove.
Arch 2—The muscles of facial expression appear as sheets of premuscle cells (laminae) that extend into the occipital, cervical and mandibular regions. Deep to each lamina is one or more branches of the facial nerve.
Caudal arches—It is difficult to separate the mesenchyme in the caudal arches. This mesenchyme collectively gives rise to the muscles of the pharynx (stylopharyngeus and pharyngeal constrictors) and larynx, which are innervated by branches of cranial nerves IX and X.
Sternocleidomastoid-trapezius muscles—These muscles become separate masses in the lateral cervical region. Each receives a separate branch of the spinal accessory nerve.
FLOOR OF THE PHARYNX
The intrinsic and extrinsic muscles of the tongue can be identified along with their innervation by the hypoglossal nerve.
B. MYOTOMIC DERIVATIONS
The myotomic derivations are indicated in Table 7–1.
C. LATERAL (PLATE) MESODERM
Somatic layer—Experimental studies on lower forms suggest that these mesenchymal cells give rise to the muscles of the limbs (see below) and also contribute to the formation of the muscles in the ventral and lateral body wall (see Table 7-1).
Splanchnic layer—The muscularis layer of the gut differentiates from the condensation around the endodermal tube of the alimentary tract. Cartilages form in the condensation around the endodermal tracheobronchial tree.
D. LIMB MESODERM
Muscles differentiate in situ from the mesenchymal cells within the limb buds.
Individual muscles first appear proximally in each limb bud.
Extensor muscles develop in the dorsal part of the limb, flexor muscles develop in the ventral part.
In the upper limb the radial nerve supplies the extensors; the musculocutaneous, median and ulnar nerves innervate the flexors.
In the lower limb the pattern is more complex with some nerves innervating both extensors and flexors. The major nerves (femoral, obturator, sciatic and its common peroneal and tibial branches) can be identified.
No significant changes are apparent in the ectoderm. The underlying mesenchymal cells produce a thick layer of loose tissue that separates the ectoderm from the deeper muscle masses.
B. MAMMARY GLAND
The major part of the mammary ridge disappears. Only a small circular area remains in the thoracic region at the level of the T-4 intercostal space. It represents the mammary gland primordium and begins to penetrate the underlying tissue (Fig. 7–1A).
The oral epithelium thickens on the maxillary and mandibular processes forming the labiodental laminae that are arranged in a U-shaped manner on the surface of each process. The laminae follow the configuration of the primordial jaws.
OPTIC CUPFig. 7–1A
The three zones of the visual layer of the retina are limited to the deep four-fifths of the optic cup where the layer is thick and is called the pars optica retinae. The superficial one-fifth is thin and is referred to as the pars caeca retinae.
The three zones of the pars optica retinae will differentiate the following manner.
The ependymal zone located next to the intraretinal space will become the photoreceptor layer of rods and cones.
The mantle zone will produce the ganglion and nuclear layers.
The marginal zone next to the optic cup cavity will form the nerve fiber layer.
The pars caeca retinae will differentiate into the iris and ciliary body, both of which contain smooth muscle.
Hyaloid blood vessels differentiate within the mesenchyme that passes through the choroid fissure into the optic cup cavity. These vessels supply the visual part of the retina and initially the lens. A delicate network of fibers, the primordial vitreous body, develops between the lens and the retina.
OPTIC STALKFig. 7–2A
The relative diameter of the optic stalk is reduced as it increases in length.
The optic stalk lumen that connects the intraretinal space with the third ventricle of the brain is reduced.
The lens cavity disappears as the deep lens epithelium becomes contiguous with the superficial lens epithelium. The deep layer forms the primary lens fibers which collectively make up the nucleus of the lens.
MESENCHYME AROUND THE OPTIC CUP
A space develops in the mesenchyme between the brim of the optic cup and the overlying ectoderm. This space becomes the anterior chamber, which separates the mesenchyme into two layers; a thin inner layer next to the lens called the iridopupillary membrane and a thick outer layer next to the ectoderm, which together give rise to the cornea.
The loose mesenchyme adjacent to the surface of the optic cup condenses to form a capsule next to the pigmented layer of the retina. At the brim of the optic cup the capsule is continuous with the cornea.
ACCESSORY STRUCTURESFig. 7–1A
Thick flaps of ectoderm, called upper and lower eyelid folds, contain mesenchymal cells and begin to grow over the surface of the eye.
The walls of the maxillonasal groove fuse to form the nasolacrimal duct that courses from the medial aspect of the eye to the primitive nasal cavity.
The extraocular muscles appear as distinct masses each receiving an innervation from either cranial nerve III, IV or VI. The ophthalmic division of cranial nerve V and the autonomic ciliary ganglion are evident in the vicinity of the eye.
INTERNAL EARFig. 7–5B
Three separate semicircular canals (anterior, posterior and lateral) develop from three flat outpouchings of the upper part of the vestibular pouch. The central area of the wall of each outpouching fuses and disappears forming a curved duct. The anterior and posterior canals join to form a common crus so that there is a total of only five crura.
The five crura are received into a large sac called the utricle that develops from the lower part of the vestibular pouch.
The endolymphatic diverticulum arises from the lower part of the utricle. Its proximal narrow segment becomes the endolymphatic duct. Its distal end near the roof of the fourth ventricle dilates to form the endolymphatic sac.
A small sac known as the saccule differentiates from the upper part of the cochlear pouch.
The cochlear duct develops from the lower part of the pouch but remains in communication with the saccule. The duct begins to spiral.
Cartilage appears in the condensation around the derivatives of the vestibular and cochlear pouches.
MIDDLE EARFig. 7–2B
As the tubotympanic recess elongates in a dorsolateral direction, its narrow proximal part becomes the auditory tube and its wide distal part forms the primitive tympanic cavity.
Mesenchymal condensations at the dorsal end of the first and second arch cartilages represent the middle ear ossicles. The condensations initially lie dorsal to the primitive tympanic cavity. The malleus and incus differentiate at the dorsal end of the first arch cartilage, the stapes differentiates at the dorsal end of the second arch cartilage.
A remnant of the second aortic arch, the stapedial artery, passes through stapes condensation accounting for the opening in its center. The artery is a transient structure.
EXTERNAL EARFig. 7–1A
The auricle begins to take shape as the auricular hillocks blend around the first branchial groove. The groove gives rise to the external acoustic meatus.
C. OLFACTORY APPARATUS
THE 18-MM EMBRYO
AGE 45 TO 47.5 DAYS
CROWN TO RUMP LENGTH 18.0 MM
Hooker-Humphrey collection 142
Streeter GL: Developmental horizons in human embryos. Description of age groups XIX, XX, XXI, XXII and XXIII. Contrib Embryol Carnegie Instn 32:133–203, 1951
A section through the tectum of the mesencephalon and edge of the metencephalon.
- The dorsal part of the mesocoele and fourth ventricle.
- The dorsal part of the middle cerebral plexus of veins in the ectomeninx.
A section through the dorsal part of the mes- and metencephalon junction.
- The rootlets of the trochlear nerve arising from the junction area.
- The thin roof of the fourth ventricle joining the alar plate of the metencephalon.
FIG. 7–6 [printable version of FIG 7-6]
A section through the middle of the mes- and metencephalon junction.
- The communication of the mesocoele with the fourth ventricle through the isthmus.
- The sulcus limitans separating the basal and alar plates.
- The three layers of the brain wall.
- The lateral part of the alar plate of the metencephalon called the rhombic lip, which gives rise to the cerebellum.
- The intra- and extraventricular portions of the rhombic lip.
A section through the ventral part of the mes- and metencephalon junction and edge of the myelencephalon.
- The minute trochlear nerve coursing ventrally at the lateral aspect of the junction region.
- The continuity of the basal plates in the junction region.
- The interpeduncular fossa separating the basis pedunculi areas.
- The narrow portion of the mesocoele, which is becoming the cerebral aqueduct.
- The roof of the fourth ventricle joining the dorsal edge of the myelencephalon.
FIG. 7–7 [printable version of FIG 7-7]
A section through the alar plate of the myelencephalon.
- A transverse section through the mesencephalon showing its major subdivisions.
- The region of the cephalic flexure containing the oculomotor nerve and the bifurcation of the basilar artery.
- The major subdivisions of the metencephalon.
- The superior cerebellar artery coursing laterally on the ventral surface of the metencephalon.
- The two subdivisions of the primitive meninx lateral to the myelencephalon: ectomeninx and endomeninx.
A section through the basal plate of the myelencephalon and caudal part of the diencephalon.
- The major subdivisions of the diencephalon and the continuity of the cerebral aqueduct in Section 5 with the third ventricle.
- The median sulcus and sulcus limitans subdividing the floor of the fourth ventricle.
- The close relationship of the endolymphatic sac to the roof of the fourth ventricle.
- The midline raphe in the ventral part of the myelencephalon produced by crossing fibers.
- The rootlets of the vagus and cranial accessory nerves that arise from the ventrolateral surface of the myelencephalon.
FIG. 7–8 [printable version of FIG 7-8]
A section through the met- and myelencephalon junction.
- The tegmentum of the pons area of the metencephalon.
- The dorsal edge of the otic capsule showing the junction of the anterior and posterior semicircular canals.
- The region of the cervical flexure containing the rootlets of the hypoglossal nerve.
- The rootlets of cranial nerve IX arising in series with cranial nerve X.
- The raphe separating the pyramidal tract areas in the ventral part of the myelencephalon.
A section through the edge of the cerebral vesicle and basal portion of the pons area.
- The three sulci in the lateral wall of the diencephalon separating it into four subdivisions.
- The root of the trigeminal nerve arising from the pons; the roots of cranial nerves VII and VIII arising from the region of the met- and myelencephalon junction.
- The common crus and its junction with the utricle of the membranous labyrinth.
- The venous sinuses in the ectomeninx.
- The spinal accessory nerve in two regions and the junction of the cranial accessory nerve with the vagus nerve.
FIG. 7–9 [printable version of FIG 7-9]
A section through the C-1 spinal ganglion at the junction of the myelencephalon and spinal cord.
- The junction of the cerebral vesicle with the diencephalon.
- The tuberal portion of the adenohypophysis on each side of the neurohypophyseal bud.
- The trigeminal ganglion with the motor root of V medially.
- The junction of the vertebral arteries to form the basilar artery.
- The junction of the lateral semicircular canal with the utricle and the relation of the vestibular ganglion to the cochlear and facial nerves.
A section through the base of the developing skull (cranial tip of notochord and parachordal cartilage).
- The vessels and cranial nerves lateral to the distal portion of the adenohypophysis.
- The three divisions of the trigeminal nerve; the ophthalmic, maxillary and mandibular nerves.
- The geniculate ganglion on cranial nerve VII and its position lateral to the saccule and cochlear duct.
- The junction of the primary head vein with the stem of the posterior cerebral plexus.
- The relation of the C-1 spinal nerve to the vertebral artery.
FIG. 7–10 [printable version of FIG 7-10]
A section through the C-2 spinal ganglion.
- The communication of the third ventricle with the lateral ventricle through the interventricular foramen.
- The ophthalmic, maxillary and mandibular branches of the trigeminal nerve and their related parasympathetic ganglia.
- The orbitosphenoid and alisphenoid of the developing skull.
- The condensation representing the stapes ossicle in the middle ear area.
- The relation of cranial nerves IX, X and XI to the internal jugular vein as they emerge from the skull.
A section through the dorsal edge of the optic cup and primitive pharynx.
- The choroid invagination into the lateral ventricle and the relation of the primordial hippocampus and its cortex to the choroid fissure.
- The junction of the optic stalk and diencephalon.
- The hypophyseal stem remnant passing through the hypophyseal cartilage.
- The mandibular nerve terminating in the muscles of mastication.
- The condensations representing the malleus and incus ossicles between the external acoustic meatus and primitive tympanic cavity.
FIG. 7–11 [printable version of FIG 7-11]
A section through the C-3 spinal ganglion and the junction of the auditory tube (first pouch) with the primitive pharynx.
- The rostral end of the diencephalon where the optic stalk arises and courses to the optic cup.
- The condensations representing the extraocular muscles dorsal to the optic cup.
- The relation of the ophthalmic and maxillary nerves to the optic cup.
- The edge of the tongue in the floor of the primitive pharynx.
- The chorda tympani branch of the facial nerve passing between the external acoustic meatus and the primitive tympani cavity.
A section through palatine fossa (second pouch) and the primitive oral cavity.
- The three subdivisions of the pallium or primordial cerebral cortex.
- The caudal part of the lateral palatine process where it is lateral to the tongue.
- The epiglottis caudal to the tongue projecting into the primitive pharynx.
- The major nerves lateral to the primitive pharynx.
- The extensor muscles of the neck.
FIG. 7–12 [printable version of FIG 7-12]
A section through the C-4 spinal ganglion and the preoptic area of the telencephalon.
- The junction of the optic stalk with the optic cup.
- The two parts of the retina: pars caeca retinae and pars optica retinae.
- The region of the falx cerebri between the cerebral vesicles.
- The first and second branchial arch cartilages.
- The three layers of the spinal cord in the midcervical region.
A section through the middle of the optic cup and laryngeal orifice.
- The zones or layers of the optic cup and its surrounding structures.
- The intrinsic muscles of the tongue flanked on each side by a lateral palatine process.
- The linguogingival groove lateral to the tongue.
- The auricle forming around branchial groove 1 (external acoustic meatus).
- The arytenoid swelling between the laryngeal orifice and the laryngeal pharynx.
FIG. 7–13 [printable version of FIG 7-13]
A section through the extrinsic tongue muscles and hyoid apparatus (second and third arch cartilages).
- The lamina terminalis and primordial olfactory bulb of the telencephalon.
- The lens epithelium.
- The eyelid folds beginning to cover the cornea.
- The parotid and submandibular buds originating from the oral cavity epithelium.
- The submandibular (parasympathetic) ganglion on the lingual branch of the mandibular nerve as it courses into the tongue.
A section through the laryngeal cartilages and caudal part of the primitive nasal cavity.
- The oronasal membrane separating the primitive oral and nasal cavities.
- The mandibular lamina located superficial to the terminal branches of the facial nerve.
- The hypoglossal nerve coursing into the tongue.
- The bifurcation of the common carotid artery.
- The roots and primary rami of a spinal nerve.
FIG. 7–14 [printable version of FIG 7-14]
A section through the C-5 spinal ganglion and primitive choana.
- The olfactory nerve coursing from the olfactory epithelium.
- The primitive choana between the lateral palatine process and the nasal septum.
- The origin of the nasolacrimal duct between the lateral nasal elevation and maxillary process.
- The tongue tip in the primitive oral cavity near the primitive rima oris.
- The caudal parathyroid bud derived from the third pouch of younger embryos.
A section through the C-6 spinal ganglion and the primitive palate.
- The vomeronasal nerve coursing from the vomeronasal organ.
- The relation of the maxillary process and nasal elevations to the primitive palate.
- The rostral (fourth pouch) and caudal (third pouch) parathyroid buds dorsolateral to the thyroid gland.
- The trachea ventral to the esophagus.
- The edge of the shoulder.
FIG. 7–15 [printable version of FIG 7-15]
A section through the greater thymic bud (third pouch) and the isthmus of the thyroid gland.
- The trabecular cartilage forming in the nasal septum.
- The infraorbital nerve in the maxillary process and the inferior alveolar nerve in the mandibular process.
- The cervical muscle lamina in the mandibular process from which the platysma develops.
- The origin of the right subclavian from the brachiocephalic artery.
- The relation of the glands, vessels and nerves to the trachea and esophagus.
A section through the C-7 spinal ganglion and clavicle.
- The nasal capsule condensation surrounding the primitive nasal cavity.
- The primordium of the first digit of the hand.
- The recurrent laryngeal nerve between the trachea and esophagus.
- The jugular lymph sac lateral to the internal jugular vein.
- The sympathetic trunk lateral to the esophagus.
FIG. 7–16 [printable version of FIG 7-16]
A section through the aortic arch and the tip of the mandibular process.
- The relation of the nasal elevations to the maxillary process.
- The primordium of the second digit of the hand.
- The clavicle joining the episternal cartilage.
- The junctions of the brachiocephalic and left common carotid arteries with the aortic arch.
- The close relation of the left vagus nerve to the aortic arch.
A section through the external nostril and cranial part of the pericardial cavity.
- The edge of the cerebral vesicle surrounded by the endo- and ectomeninx.
- The relation of the nasal elevations to the external nostril.
- The oblique position of the aortic arch.
- The muscles and nerves around the scapula.
- The upper trunk of the brachial plexus passing through the scalene muscles.
FIG. 7–17 [printable version of FIG 7-17]
A section through the C-8 spinal ganglion and the edge of the frontal and nasal regions.
- The ascending and descending aortas that are continuous with the aortic arch in Section 24.
- The left recurrent laryngeal nerve coursing to the tracheoesophageal groove.
- The head of the humerus deep to the deltoid muscles.
- The phrenic nerve lateral to the precardinal vein.
A section through the T-1 spinal ganglion and cranial edge of the atria.
- The ascending aorta to the right of the pulmonary trunk from which it is separated by the truncus septum.
- The ductus arteriosus connecting the pulmonary trunk and aorta.
- The left vagus nerve lateral to the ductus arteriosus with its branch, the recurrent laryngeal nerve, medial.
- The sympathetic trunk medial to the head of the first rib.
- The relative position of the extensor and flexor muscles in the forearm.
FIG. 7–18 [printable version of FIG 7-18]
A section through the pulmonary semilunar valve.
- The infundibulum of the right ventricle below the pulmonary semilunar valve and the right and left pulmonary arteries arising from the pulmonary trunk above the valve.
- The pulmonary arteries passing through the cranial part of the mesocardium.
- The origin of the left recurrent laryngeal nerve from the left vagus nerve between the aorta and left pulmonary artery.
- The brachial plexus and axillary artery, vein and lymph sac lateral to the first rib.
- The primordium of the third digit of the hand.
A section through the T-2 spinal ganglion.
- The ascending aorta dorsal to the infundibulum of the right ventricle at the level of the truncoconal junction.
- The transverse sinus that separates the cranial (arterial) part of the mesocardium (see Section 27) from the caudal (venous) part (see Section 31).
- The right pulmonary artery passing to the right between the ascending aorta and trachea.
- The anterior serratus muscle attaching the scapula to the ribs.
- The sternal bar at the ventral end of rib 2.
FIG. 7–19 [printable version of FIG 7-19]
A section through the aortic semilunar valve.
- The conus septum separating the aortic semilunar valve from the infundibulum of the right ventricle.
- The cranial edge of the pleural cavity on the right between the rib and superior vena cava.
- The aorta to the left of the esophagus.
- The phrenic nerve dorsolateral to the pericardial cavity and the vagus nerve lateral to the trachea.
- The elbow joint area between the humerus and ulna.
A section through the T-3 spinal ganglion and the tracheal bifurcation.
- The cranial part of the left ventricle dorsal to the right ventricle from which it is separated by the membranous part of the interventricular septum.
- The septum secundum between the right and left atria and the ostium secundum through which the two atria communicate.
- The entrance of the superior vena cava into the pericardial cavity and its junction with the sinus venosus.
- The phrenic nerve coursing through the thin pleuropericardial membrane that separates the pericardial and pleural cavities.
- The primordium of the fourth digit of the hand.
FIG. 7–20 [printable version of FIG 7-20]
A section through the T-4 spinal ganglion and the right atrioventricular (A-V) canal.
- The trabeculae carneae cordis in the wall of the right ventricle and the musculi pectinati in the right atrial wall.
- The sinus venosus between the septum secundum and right venous valve.
- The entrance of the left common cardinal vein into the pericardial cavity, then passing toward the right, dorsal to the heart.
- The right pulmonary artery coursing into the lung ventral to the right primary bronchus.
- The primordium of the mammary gland and fifth digit of the hand.
A section through the T-5 spinal ganglion and the right and left atrioventricular valves.
- A papillary muscle attaching a leaf of the right atrioventricular valve to the wall of the right ventricle.
- The septum primum caudal to the ostium secundum (see Section 31) and left of the septum secundum.
- The right venous valve joining the fused endocardial cushions.
- The left atrioventricular canal connecting the left atrium with the left ventricle.
- The esophagus passing ventral to the aorta.
FIG. 7–21 [printable version of FIG 7-21]
A section through the T-6 spinal ganglion and caudal edge of the ventricles.
- The apex of the left ventricle and three layers of the heart wall.
- The continuation of the left common cardinal vein in Section 32 with the left sinus horn (coronary sinus) that empties into the sinus venosus.
- The pulmonary veins that course cranially from the lungs through the caudal part of the mesocardium in Section 32 before they empty into the left atrium.
- The phrenic nerves in the pleuropericardial membrane dorsolateral to the heart.
- The fissures that separate the lungs into lobes.
A section through the T-7 spinal ganglion and the cranial edge of the liver.
- The septum transversum separating the pericardial and peritoneal cavities and the pleuroperitoneal membrane separating the pleural and peritoneal cavities.
- The right phrenic nerve lateral to the inferior vena cava.
- The vagus nerves forming a plexus around the esophagus.
- The dorsal mesoesophagus separating the pleural cavities between the aorta and esophagus.
- A ramus communicans connecting the sympathetic trunk with an intercostal nerve.
FIG. 7–22 [printable version of FIG 7-22]
A section through the T-8 spinal ganglion and caudal part of the pericardial cavity.
- The phrenic nerves approaching the septum transversum part of the diaphragm.
- The inferior vena cava near the bare area of the liver.
- The lower lobes of the lungs separated by the esophagus.
- The azygos line vein lateral to the aorta.
- The origin of a posterior intercostal artery from the aorta.
A section through the pleuroperitoneal canal and base of the lungs.
- The efferent veins of the liver draining blood from the hepatic sinusoids to the hepatocardiac segment of the inferior vena cava.
- Muscle forming in the diaphragm near the esophagus.
- The communication of the pleural cavity with the peritoneal cavity through the pleuroperitoneal canal.
- The two subdivisions of the deep muscles of the back: erector spinae muscle group and transversospinalis muscle group.
FIG. 7–23 [printable version of FIG 7-23]
A section through the T-9 spinal ganglion and the cranial edge of the mesonephros.
- Efferent veins draining into the ductus venosus.
- The junction of the ductus venosus with the hepatocardiac segment of the inferior vena cava.
- The relation of the vagal trunks to the esophagus as it passes through the diaphragm in Section 36.
- The cranial edge of the suprarenal gland, lateral to the caudal edge of the pleural cavity.
A section through the T-10 spinal ganglion and the esophageal-gastric junction.
- The dorsal mesogastrium and the two divisions of the ventral mesogastrium: lesser omentum and falciform ligament.
- The position of the vagal trunks in relation to the stomach.
- The cranial edge of the mesonephric duct and indifferent gonad.
- The celiac ganglion ventral to the aorta between the suprarenal glands.
FIG. 7–24 [printable version of FIG 7-24]
A section through the epiploic foramen.
- The two subdivisions of the peritoneum.
- The growth of the stomach to the left.
- The communication of the lesser sac with the greater sac through the epiploic foramen.
- The minute female (paramesonephric) duct ventral to the mesonephric duct.
A section through the T-11 spinal ganglion and the cranial edge of the spleen.
- The junction of the portal vein with the ductus venosus.
- Afferent veins arising from the ductus venosus and bringing blood to the hepatic sinusoids.
- The splenic artery (branch of the celiac artery) approaching the dorsal mesogastrium.
- The hepatic artery (branch of the celiac artery) coursing to the liver with the portal vein.
- The suprarenal medulla medial to the cortex and its close relation to the celiac plexus.
FIG. 7–25 [printable version of FIG 7-25]
A section through the middle of the lesser sac.
- The four layers of the stomach wall.
- The junction of the cystic and hepatic ducts to form the bile duct.
- The splenic vessels coursing through the dorsal mesogastrium.
- The superior mesenteric artery passing into the dorsal mesoduodenum with the subcardinal segment of the inferior vena cava on its right side.
- The centrum and neural arch portions of a vertebra.
A section through the T-12 spinal ganglion and gastroduodenal junction.
- The gall bladder on the caudal surface of the liver and drained by the cystic duct.
- The junction of the splenic and superior mesenteric veins to form the portal vein in Section 41 between the dorsal and ventral pancreas.
- The relation of the splenic vein to the dorsal pancreas.
- The superior mesenteric ganglion ventral to the aorta and medial to the suprarenal medulla.
- A mesonephric capsule and tubule.
FIG. 7–26 [printable version of FIG 7-26]
A section through the cranial edge of the metanephros.
- The junction of the umbilical vein and the ductus venosus at the surface of the liver.
- The fore- and midgut junction on the duodenum.
- The duodenum assuming a position dorsal to the superior mesenteric vessels.
- The intersubcardinal anastomosis that will form the left renal vein.
- The three muscles in the lateral abdominal wall.
A section through the L-1 spinal ganglion and the cranial edge of the junction of the umbilical cord and the ventral body wall.
- The umbilical vein leaving the umbilical cord and the junction of the superior mesenteric veins.
- The cranial major calyx of the primitive renal pelvis seen in Section 45.
- The superior mesenteric vessels in the dorsal mesentery passing to the right and ventral to the caudal segment of the duodenum.
- The subcardinal vein segment that will form the gonadal vein and the inferior vena cava segment that develops from the anastomosis between the right supra- and subcardinal veins.
- The inferior mesenteric ganglion ventral to the aorta.
FIG. 7–27 [printable version of FIG 7-27]
A section through the L-2 spinal ganglion and edge of the thigh.
- The extension of the peritoneal cavity into the umbilical cord to form the umbilical coelom that receives the herniated midgut.
- The cranial limb of the midgut to the right of the caudal limb as a result of the counterclockwise rotation.
- The mid- and hindgut junction near the midline in the dorsal mesentery.
- The origin of the inferior mesenteric artery from the aorta.
- The primitive renal pelvis medial to the metanephros where it gives rise to the caudal major calyx.
A section through the cranial edge of the umbilical arteries and allantois.
- The herniated portion of the cranial and caudal limbs of the midgut separated by the superior mesenteric vessels.
- The junction of the duodenum and the cranial limb of the midgut, caudal to the liver.
- The renal capsule and collecting tubules of the metanephros, lateral to the metanephric duct.
- The supracardinal segment of the inferior vena cava.
- The external iliac artery entering the thigh as the femoral artery.
FIG. 7–28 [printable version of FIG 7-28]
A section through the aortic bifurcation and the bladder portion of the urogenital sinus.
- The dilation on the herniated caudal limb of the midgut that represents the cecum.
- The umbilical arteries and allantois entering the umbilical cord.
- The mesonephric duct coursing toward the urogenital sinus.
- The caudal edge of the junction between the duodenum and midgut loop.
- The iliac lymph sac around the external iliac vessels.
A section through the L-3 spinal ganglion and the caudal edge of the metanephros.
- The appendix represented as an outpouching of the caudal limb of the midgut.
- The caudal edge of the junction of the umbilical cord and the ventral body wall.
- The primordium of the first digit of the foot.
- The obturator nerve coursing medial to the pubis of the developing pelvic bone.
- The three terminal branches of the aorta: right and left common iliac and median sacral arteries.
FIG. 7–29 [printable version of FIG 7-29]
A section through the L-4 spinal ganglion and the cranial edge of the genital tubercle.
- The junction of the mesonephric duct and the urogenital sinus.
- The metanephric duct coursing ventrally to join the urogenital sinus.
- The inferior vena cava segment formed by the anastomosis of the post-cardinal veins.
- The ilium and ischium of the developing pelvic bone.
- The knee joint area between the femur and tibia.
A section through the pelvic portion of the urogenital sinus and the caudal edge of the peritoneal cavity.
- The pelvic portion of the urogenital sinus deep to the genital tubercle.
- The junction of the hindgut and the rectum.
- The sacral plexus and sciatic nerve coursing into the thigh.
- Parasympathetic neuroblasts in the lateral pelvic wall.
- The primordium of the second digit of the foot.
FIG. 7–30 [printable version of FIG 7-30]
A section through the L-5 spinal ganglion and the phallic portion of the urogenital sinus.
- The superior mesenteric artery leaving the dorsal mesentery to become the vitelline artery.
- The urogenital part of the cloacal membrane between the urogenital folds.
- The phallic portion of the urogenital sinus separated from the exterior by the urogenital part of the cloacal membrane.
- The urorectal septum separating the urogenital sinus and rectum.
A section through the urogenital fold and groove.
- The labioscrotal swelling lateral to the urogenital fold.
- The urorectal septum between the rectum and cloacal membrane.
- The primordium of the third digit of the foot.
FIG. 7–31 [printable version of FIG 7-31]
A section through the S-1 spinal ganglion.
- The caudal edge of the anal fold lateral to the proctodeum.
- The junction of the rectum and the anal part of the cloacal membrane.
A section through the tip of the tail bud remnant.
- The caudal end of the sympathetic trunk.
- The relative position of the extensor and flexor muscles in the leg.
FIG. 7–32 [printable version of FIG 7-32]
A section through the caudal edge of the herniated midgut.
- The primordium of the fourth digit of the foot.
- The median sacral artery coursing into the tail bud remnant.
- The four plates of the spinal cord in oblique section.
A section through the S-2 spinal ganglion and the ventral primary rami of remaining spinal nerves (S-3 through coccygeal).
- The primitive filum terminale ending in the tail bud remnant where the coccyx will form.
- The notochord in longitudinal section.
- The centrum and intervertebral disc of the vertebral column.
- The primordium of the fifth digit of the foot.
FIG. 7–33 [printable version of FIG 7-33]
A section through the S-3 and S-4 spinal ganglia.
- An oblique section of the spinal cord showing its three layers.
- The junction of the filum terminale and the spinal cord.
A section through the S-5 and coccygeal spinal ganglia.
- A longitudinal section of the neural canal.
- The caudal edge of the foot.
FIG. 7–34 [printable version of FIG 7-34]