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7 1/2 week embryo, 7.5 week embryo, seven and a half week embryo

Interactive Prenatal Development Timeline

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	Unit 1: The First Week
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(Day 0 - Day 1): Carnegie Stage 1 Embryonic Period Begins (Day 1 - Day 1): Fertilization resulting in zygote formation (Day 2): Carnegie Stage 2 Activation of the genome Blastomeres begin rapidly dividing (Day 2 - Day 2): Early pregnancy factor (EPF) (Day 3): Compaction Embryo is spherically shaped and called a morula comprised of 12 to 16 blastomeres (Day 4): Carnegie Stage 3 Embryonic disc See where the back and chest will be (Day 5): Hatching Inner cell mass (Day 6): Carnegie Stage 4 The embryo implants into wall of womb (1 week): Carnegie Stage 5a Amnioblasts present; amnion and amniotic cavity formation begins Chorion Chorionic cavity Extraembryonic mesoblast (or mesoderm) Hypoblast & epiblast Placenta begins to form Solid synctiotrophoblast & cytotrophoblast Less Events...

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	Unit 3: 2 to 3 Weeks
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(2 weeks, 1 day): 3 germ layers Cloacal membrane Prechordal plate Primitive groove Rostral-caudal orientation (2 weeks, 2 days): Carnegie Stage 7 Erythroblasts in yolk sac Three types of blood-forming cells in yolk sac Primordial germ cells Allantoic diverticulum Allantoic diverticulum Amnion with two cell layers Secondary villi (2 weeks, 3 days): Notochordal process (2 weeks, 4 days): Carnegie Stage 8 Foregut, midgut, and hindgut Uteroplacental circulation well established Prechordal plate with 1 retinal field Brain is first organ to appear Neural ectoderm Neural groove and neural folds Notochordal and neurenteric canals Notochordal plate Connecting stalk Primitive pit (or notochordal pit) (2 weeks, 5 days): Prechordal plate with 2 retinal fields (2 weeks, 6 days): Carnegie Stage 9 Numerous blood islands in umbilical vesicle Septum transversum (primitive diaphragm) Foregut Pharyngeal pouch 1 Stomodeum forming Blood vessels emerge simultaneously in umbilical vesicle, embryo proper, amnion, and connecting stalk Cardiogenic plate, cardiac jelly, myocardial mantle, and endocardial plexus Dorsal aortae (paired) First pair of aortic arches Paired pericardial cavities Paired tubular heart Otic disc Caudal eminence Hindbrain with four rhombomeres Isthmus rhombencephali demarcates midbrain and hindbrain Mesencephalon (or midbrain) Neural crest Neural groove deepens substantially Primary neuromeres Three main divisions of brain Cephalic and caudal folds First somites Primitve streak reaches neurenteric canal Somites: 1 - 3 pairs Less Events...

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	Unit 5: 4 to 5 Weeks
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(4 weeks, 4 days): Carnegie Stage 14 Thymus Parathyrogenic zones Thyroglossal duct Thyroid pedical lengthens Dorsal contour develops depression at level of sclerotomes 4 and 5 Muscular plates between upper and lower limb buds Glomerular capsules, partially vascularized Mesonephric corpuscle Metanephrogenic cap emerges from ureteric bud Ureteric buds Angiogenesis within peri-esophageal mesenchyme Epiploic foramen Lesser sac (omental bursa) Small intestine forming coils Tongue: Hypopharyngeal eminence Arytenoid swellings (right and left) Capillary network surrounds pulmonary mesenchyme Epithelial lamina of larynx Lungs: Right and left primary (or main stem) bronchi Mesenchyme covering esophagus and respiratory tree separates Mesenchyme surrounds bronchi Pleura (mesothelium) surrounds part of mesenchyme Right main bronchus longer than left Atria walls thin, ventricle walls thick and trabeculated Atrioventricula cushions not fused Conotruncal ridges or cushions (remnants of cardiac jelly) Epicardium Outflow tract still with one lumen Posterior communicating arteries Pulmonary arch (sixth aortic arch) forms from aorta and aortic sac Pulmonary capillary network fed by pulmonary arteries, drain into left atrium Sinu-atrial (SA) node Superior mesenteric artery and vein Upper limb buds with early marginal blood vessel Brachial plexus Cervical plexus Lens and retina invaginate to form optic cup Primordium of cochlear duct Spinal nerves reach muscle primordia Upper limb buds innervated D1 and D2 no longer identifiable within diencephalon 75% of midbrain covered by marginal layer Adenohypophysis All 16 secondary neuromeres Brain enlarges 50% since Carnegie Stage 13 Brain: Cerebral hemispheres appear and begin rapid growth Brain: Lateral ventricles Cerebellar primordium found in alar plate of rhombomere 1 Cerebellum with intermediate and ventricular layers Corpora striata primordia connected by commissural plate Cranial nerve 3 Di-telencephalic sulcus Dorsal and ventral thalami Dorsal funiculus Hypoglossal nerve roots unite Mamillary region Medial and lateral longitudinal fasciculi Median ventricular eminence Pontine flexure Preoptic sulcus extends between optic evaginations Preoptico-hypothalamo-tegmental tract Rami communicantes Rhombic lip Spinal cord wall with three zones: ventricular (ependymal) zone, mantle (intermediate) zone, and marginal zone Subthalamus with medial striatal ridge emerging Synencephalon Tegmentum Tentorium cerebelli, medial portion Terminal-vomeronasal crest contacts brain (olfactory area) Torus hemisphericus (TH) Velum transversum Ventral longitudinal fasciculus Ventral segment of hyoid arch subdivides (4 weeks, 5 days): Carnegie Stage 15 Gonad framework found in coelomic epithelium Primary urogenital sinus Thyroid detached from epithelium of pharynx in some embryos Lower limb bud rounded proximally and tapered distally Mesenchymal skeleton in upper and lower limbs Right and left neural processes Vertebrae well defined Vertebral centra Ureteric bud extends to pelvis of the ureter Bladder and rectum are separating caudal to ureters Dense mesenchyme surrounds much of gastrointestinal tract Esophagus elongating and passing dorsal to carina and between main stem bronchi Gall bladder and cystic duct Intestinal loop Liver: Hepatic ducts Ventral pancreas appears as an offshoot of the cystic duct Lobar bud swellings denote areas of secondary bronchi Remnants of coelomic epithelium forming visceral pleura Atrioventricular cushions apposed Blood flow divided into right and left streams through atrioventricular canal, ventricles, outflow tract, and aortic sac Blood vessels penetrate diencephalon Capillary plexus surrounds esophagus Capillary plexus surrounds lung buds Cardiac mesenchyme surrounds ventricles and outflow tract Common pulmonary vein drains pulmonary plexuses into left atrium Coronary arteries (terminal end) Foramen secundum begins in septum primum Left ventricle with thicker walls and greater volume than right Semilunar cusps Capsule present around lens Corneal epithelium overlying optic cup Ear: Endolymphatic duct External ear primordia emerges from caudolateral portion of mandibular arch Lens body now present containing some lens fibers Lens vesicles beneath surface Lens vesicles closed, pores absent Lower limb buds innervated Optic chiasm Optic stalk Primordium of antitragus emerges from ventral subsegment of hyoid arch Utriculo-endolymphatic fold Face: Lateral and medial nasal processes bilaterally Lateral nasal processes along dorsolateral lip of nasal pits Pigment in retina (external layer of optic cup) Adult lamina terminalis Amygdaloid area Cerebellar plate Cerebellum with marginal layer Fibers of dorsal funiculus reach level of C1 First axodendritic synapses in cervical spinal cord First nerve fibers Geniculate and vestibulocochlear ganglions separating Habenular nucleus Habenulo-interpeduncular tract Hypothalamic sulcus Hypothalamus Lateral striatal ridge (derived from telencephalon and comprised mainly of neostriatum) Lateral ventricular eminence Locus caeruleus Longitudinal zones in diencephalon Marginal layer throughout most of diencephalon Material for sympathetic trunks scattered in cervical region Median striatal ridge (paleostriatum) Mesencephalic tract of CN 5 Most cranial nerves seen Olfactory fibers reach brain Optic groove (also called preoptic recess) Postoptic recess Primary meninx surrounds most of brain Primordium of epiphysis Rhombomeres still identifiable Sclerotomic material around notochord (rhombomere D level) Superior colliculi and its commissure Superior medullary velum Supramamillary commissure Synapses among motor neurons in spinal cord Tectobulbar tract Tentorium Third ventricle Trigemino-cerebellar tract Trochlear nerve root and decussation Hand plate emerges from distal upper limb bud (5 weeks): Arytenoid and epiglottal swellings Lobar pattern mimics adult pattern T-shaped laryngeal inlet Pacemaker cells Less Events...

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	Unit 7: 6 to 7 Weeks
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(6 weeks, 2 days): Carnegie Stage 18 Angiogenesis begins inside gonads Gonad grows into oval shape with irregular surface Ostium (abdominal) of uterine tube at rostral end of paramesonephric duct (in female embryos) Paramesonephric duct forms from rostral end of mesonephric duct Testicular cords in gonads of male embryos Testicular cords in male gonad Elbow regions sometimes identifiable Embryo with cervical and lumbar flexures Embryo with dorsal concavity Finger rays with early interdigital notching Humerus: Chondrocytes in phases one through three Primordium of cartilage within nasal septum Toe rays sometimes present Stapedius muscle "Common excretory duct is disappearing" Cloacal membrane ruptures (stages 18-19) Primordia of secretory tubules Esophagus with muscular and submucous coats Submandibular gland primordia Bronchial tree with subsegmental buds Bronchial tree with well established segmental bronchi Aortic and pulmonary valves assuming shape of a cup Inferior vena cava Interventricular septum: membranous part begins forming Mesenchyme ridges in place of future mitral and tricuspid valves Origin of left coronary artery Pulmonary and aortic blood flows completely separate Secondary interventricular foramen sometimes closing (stage 18-21) interventricular septum Septum secundum and foramen ovale (stages 18-21) Superior vena cava, inferior vena cava, and sinus venosus collecting all venous blood Bucconasal membrane Bucconasal membrane detaches opening up nasal airway Choanae Crus commune Crus helicis forming from auricular hillocks two and three (from mandibular arch) Ethmoidal epithelium emerges from upper medial nasal wall Frontonasal angle (marks location of future nasal bridge) Mesenchyme thickenings mark beginning of "sclera and its muscular attachments" Nasal fin splits forming choanae and bucconasal membrane Nasal tip emerges Nasolacrimal duct begins as epithelial strand emanating from nasomaxillary groove Nerve fibers in retina Olfactory bulb sometimes with olfactory ventricle One to three semicircular ducts Optic fibers Primary lens fibers filling lens vesicle cavity Retina's outer lamina heavily pigmented Semicircular ducts form in order: anterior, posterior, and lateral Vomeronasal nerve and ganglion Vomeronasal organ marked by groove and located in fold of lower medial nasal wall Ear: Stapes primordium surrounds stapedial artery Eyelid folds sometimes present Nostrils, nasal wings, and nasal septum easily seen Adenohypophysis no longer open to pharyngeal cavity Archistriatum Brain: Dentate nucleus in internal cerebellar swellings Brain: Pineal recess emerges representing anterior lobe of epiphysis Cerebrospinal fluid production begins Choroid folds in fourth ventricle Choroid plexuses in fourth and lateral ventricles Corpus striatum much larger extending to preoptic sulcus; has subtle groove External cerebellar swellings contain future flocculus Four amygdaloid nuclei Hippocampus reaches olfactory region Neurohypophysis walls are folded Nucleus ambiguus of the vagus (CN10) Prosencephalic septum Red nucleus Substantia nigra Supraoptic commissure (6½ weeks): Volar pads on palms (6 weeks, 5 days): Carnegie Stage 19 Greater thymic bud Cheeks form by merging of maxillary and mandibular processes Mammary gland primordium Mammary ridge disappears leaving only mammary gland primordium Female duct Gonads extend from levels T-10 to L-2 Rete ovarii (in female embryos) Rete testis begins emerging from seminiferous cords (Stage 19-23) (in male embryos) Tunica albuginea in male embryos Suprarenal gland: Cortex Suprarenal gland: Medulla populated by prechromaffin cells Beginnings of occipital and sphenoid bones Bilateral cartilaginous sternal bars tie ribs together; sternal bars join cranially to form the episternal bar in the midline Cartilage within otic capsule envelops semicircular canals and cochlear duct Cartilaginous styloid process Ear: Cartilaginous malleus, incus, and stapes (the middle ear ossicles) Ectomeninx covers lateral and dorsal surfaces of brain (laying the foundation for the flat bones of the skull) Humerus, radius, and ulna Intervertebral discs form from caudal condensed portion of sclerotomes Ischium and illium Labiodental lamina: Inner dental lamina and outer labiogingival band Laryngeal cartilages Medial skull cartilages: Parachordal, hypophyseal, and trabecular Orbitosphenoid cartilage located within ectomeninx near optic stalk Ossification begins in maxilla (stages 19 -20) Rib primordia become cartilaginous Ribs each have an identifiable head and shaft Scapula and clavicle Sternum: Episternal cartilage created from fusion of right and left sternal bars Tibia and fibula Tracheal cartilages U-shaped labiodental lamina form along upper and lower oral cavity Vertebral column represented by cartilaginous centrum, neural arch, and short tranverse process Deltoid muscle Esophagus: muscularis layer adjacent to esophageal plexus External and internal abdominal oblique muscles Gluteal muscle group Iliopsoas muscles Infrahyoid muscles Internal intercostal muscles Levator scapulae muscle Limb extensor muscles located dorsally Limb flexor muscles located ventrally Longus and semispinalis cervicis muscles Midgut: Muscularis Muscle tissue formimg around phrenic nerve within septum transversum portion of diaphragm Pectoralis major muscles Pharyngeal constrictor muscle Pharyngeal constrictor muscles Platysma muscle Premuscle mass of the muscles of mastication innervated by mandibular nerve Quadratus lumborum muscle Rectus abdominis muscle Rectus posterior and semispinalis capitis muscles Rhomboid and scalene muscles Serratus anterior muscles Splenius and longissimus muscles Sternocleidomastoid and trapezius muscles distinct and innervated by sparate branches of spinal accessory nerve (CN XI) Thenar and hypothenar eminences Tongue forms from swellings in floor of pharynx Tongue: Extrinsic muscles identifiable Tongue: Intrinsic muscles identifiable Transversospinal and erector spinae muscle groups Upper limb flexors innervated by musculocutaneous, median, and ulnar nerves Major calyces, cranial and caudal, with collecting tubules within metanephrogenic mass Mesonephros extends from T-9 to L-3 Metanephros extends from T-12 to L-2 Renal capsule covers distal collecting tubules Renal vesicles form in part of metanephros Ureter forms from "proximal segment of metanephric diverticulum" Urogenital sinus comprised of three parts: Bladder, pelvic, and phallic portions Anal folds adjacent to anal membrane Anal membrane Common coelomic cavity divides into peritoneal, pericardial, and pleural cavities Duodenum: "Assumes the shape of an arc" Greater omentum Lateral palatine process Liver: rapid growth, right side greater than left Median mandibular groove disappears as mandibular processes merge in midline Palatine fossa (from pharyngeal pouch 2) Primitive oral cavity Primitive palate (or intermaxillary segment) Primitive rima oris replaces stomodeum Retropharyngeal space Stomach wall layers: Mucosa, submucosa, muscularis, and serosa Submandibular and parotid gland buds Submandibular gland duct Bronchial tree: First generation of subsegmental bronchi complete Bronchopulmonary buds (tertiary branches) Glottis, primitive Lung sac, right: Oblique and horizontal fissures define upper, lower, and middle lobes Lung sac: Apex and base Lung, left: Oblique fissure defines upper and lower lobes "Septum primum fuses with endocardial cushions" obliterating ostium primum and creating the ostium secundum Apex of left ventricle Brachiocephalic veins, right and left Circulus arteriosus (Circle of Willis) complete Common carotid arteries Ductus arteriosus External iliac and femoral arteries Iliac lymph sac Intercostal and subcostal arteries Internal thoracic artery and costocervical trunk Left subclavian artery feeds left axillary and vertebral arteries and left thyrocervical trunk Mesenteric lymph sac Mesonephric artery feeds mesonephros, gonads, and suprarenal glands Origin of right coronary artery Paired common iliac veins feed inferior vena cava Papillary muscles Pontine, superior cerebellar, and anterior and posterior interior cerebellar arteries replace myelencephalic and metencephalic arteries Primitive cavernous sinus drains primitive maxillary and supraorbital veins Primitive marginal sinus drains diencephalon Primitive tentorial sinus drains cerebral vesical Primitive transverse and sigmoid sinuses Pulmonary arteries (right and left) Right subclavian artery originates from brachiocephalic artery and feeds right thyrocervical trunk and axillary and vertebral arteries Splenic vein Tricuspid and mitral valves Anterior chamber between iridopupillary membrane and thickened ectoderm Auditory tube and primtive tympanic cavity form from tubotympanic recess pharyngeal pouch 1) Choana Cochlear duct tip grows upward Cochlear nuclei External auditory (or acoustic) meatus Glossopharyngeal nerve (CN IX) innervates stylopharyngeus premuscle mass Linguogingival groove Nasolacrimal duct forms from maxillonasal groove Nasolacrimal ducts extend from medial eyes to primitive nasal cavity Nerve fibers begin extending from retina Oculomotor (CN III), trochlear (CN IV), and abducens (CN VI) nerves innervate individual premuscle masses around optic cup Optic fibers enter chiasmatic plate Primitive nasal cavity Primordial vitreous body Saccule and cochlear duct Three semicircular ducts or canals Utricle, endolymphatic duct, and endolymphatic sac Eyelids: Upper and lower lids present and growing Adenohypophysis: Lateral lobes of pars tuberalis Adenohypophysis: Pars intermedia emerging Brain: Internal capsule formation underway Celiac, superior mesenteric, and inferior mesenteric preaortic ganglia Cerebral aqueduct Cerebral hemispheres cover half of diencephalon Esophageal plexus formed by vagal nerves (CN X) Facial nerve (CN VII) branches: Chorda tympani, greater petrosal, posterior auricular, and digastric Facial nerve (CN VII) reaches cervicomandibular region Ganglion of nervus terminalis Globus pallidus externus in the diencephalon Habenular commissure Hippocampus: Gyrus dentatus Hypoglossal nerve (CN XII) innervate separating tongue muscles Intermediate layer in dorsal thalamus Interpeduncular fossa Lateral recesses in fourth ventricle Lemniscal decussation Lower limb nerves (femoral, obturator, sciatic, common peroneal, and tibial) identifiable Medial accessory olivary nucleus Neurohypophyseal bud Nuclei of forebrain septum Nucleus accumbens Occipital pole of cerebral hemispheres Optic stalk with barely discernible lumen Paraphysis marks dividing line in roof between telencephalon and diencephalon Primitive filum terminale Radial nerve innervates upper limb extensors Rhombomeres no longer distinguishable Subcommissural organ Superior, middle, and inferior cervical ganglia Trigeminal nerve (CN V) with opthalmic, maxillary, and mandibular divisions reach their destinations Vagal trunks, anterior and posterior, extending into abdomen Zona limitans intrathalamica between dorsal and ventral thalami (6 weeks, 6 days): Cloacal membrane ruptures (7 weeks): Head rotates Ovaries Bronchopulmonary segments complete The heart has four chambers and is nearly complete. The heart rate peaks at 165 to 170 beats per minute. Crown-heel length 2.2 cm Less Events...

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	Unit 9: 8 to 9 Weeks
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(8 weeks, 1 day): Fetal Period Begins Humerus: Bone marrow replaces cartilage (8 weeks, 1 day - 9 weeks): Anal canal patent (8½ weeks): Eyelids completely fused Neurons synapse in cerebral cortex (marginal zone) (9 weeks): Drinking fluid is becoming routine Sucking the thumb External capsule Olivary nucleus with five components (9 weeks - 10 weeks): Larynx recanalizes Less Events...

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	Unit 11: 10 to 11 Weeks
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(10½ weeks): Volar and plantar pads regress (11 weeks): Intermediate layer Intestines absorb water & glucose Small intestine lined with villi Crown-heel length

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	Unit 13: 3 to 4 Months
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(13 weeks): Teeth are growing Crown-heel length 15 cm (14 weeks): Girls move their jaws more than the boys do Cerebellum resembles adult structure Crown-heel length 17 cm (15 weeks): Crown-heel length 19.5 cm (16 weeks): Quickening Colon lined with villi Canalicular stage begins Crown-heel length 21 cm Less Events...

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	Unit 15: 5 to 6 Months
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(21 weeks): Periderm disappears Stratum corneum (22 weeks): Behavioral states (23 weeks): Brain weight 100 grams (24 weeks): Crown-heel length 34.5 cm

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	Unit 17: 7 to 8 Months
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(30 weeks): Head circumference 30 cm (32 weeks): Lower esophagus muscles functional Crown-heel length 45 cm

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	Unit 19: 9 Months to Birth
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(37 weeks): Fetus drinks an estimated 15 oz (or 450cc) of amniotic fluid/day (38 weeks): Heart beats 54 million times before birth Spinal cord ends at third lumbar vertebrae Brain weight 350 grams Crown-heel length 50 cm Head circumference 35 cm Time to be born!

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Fertilization

Week 1 ends

Play Movie - 1 to 2 weeks, implantation and HCG

Week 2 ends

Play Movie - 2 to 3 week embryo, amnion, germ layer

Week 3 ends

Play Movie - 3 to 4 week embryo, folding of the embryo

Week 4 ends

Play Movie - 4 to 5 week embryo, The Heart in Action

Week 5 ends

Play Movie - 5 to 6 week embryo, hand plates develop

Week 6 ends

Play Movie - 6 to 7 week embryo, hand movement

Week 7 ends

Play Movie - 7 to 8 week embryo, fingers, toes

Week 8 ends

Play Movie - 8 to 9 week fetus, reflexive movement

Week 9 ends

Play Movie - 9 to 10 week fetus, sucking, swallowing

Week 10 ends

Play Movie - 10 to 11 week fetus, eyes roll, yawns

Week 11 ends

Play Movie - 11 to 12 week fetus, nails, fingerprints

Month 3 ends

Play Movie - 3 to 4 month fetus, the rooting reflex

Month 4 ends

Play Movie - 4 to 5 month fetus, bronchial tree

Month 5 ends

Play Movie - 5 to 6 months fetus, responds to sound

Month 6 ends

Play Movie - 6 to 7 month fetus, response to light

Month 7 ends

Play Movie - 7 to 8 month fetus, behavioral states

Month 8 ends

Play Movie - 8 to 9 month fetus, firm hand grasp

Month 9 ends

Birth

1175 events on this timeline

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	Unit 2: 1 to 2 Weeks
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(1 week, 1 day): Positive pregnancy test Bilaminar embryonic disc (1 week, 2 days): Carnegie Stage 5b Corpus luteum of pregnancy Cells in womb engorged with nutrients Exocoelomic membrane Isolated trophoblastic lacunae Yolk sac Embryonic disc 0.1 mm diameter (1 week, 4 days): Carnegie Stage 5c Intercommunicating lacunae network Longitudinal axis Trophoblastic vascular circle (1 week, 5 days): Implantation complete Primary umbilical vesicle forms from extraembryonic endoderm (or parietal endoderm) Embryonic disc diameter: 0.15 to 0.20 mm (1 week, 6 days): Carnegie Stage 6 Blood islands in umbilical vesicle Allantois Angiogenesis in chorionic mesoblast Blood vessels in villi Connecting stalk Primordial blood vessels Amnion with single cell layer Chorionic villi (2 weeks): Embryonic epiblast gives rise to primitive streak and embryonic ectoderm Primitive node Yolk sac Yolk sac Less Events...

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	Unit 4: 3 to 4 Weeks
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(3 weeks, 1 day): Carnegie Stage 10 Thyroid primordium emerges from floor of pharynx Cloaca Liver: Hepatic plate (endoderm) Midgut emerging Oropharyngeal membrane Pharyngeal arches 1 and 2 Pharyngeal cleft 1 Second pharyngeal cleft and pouch Pharyngeal groove and ridge with laryngotracheal sulcus Respiratory outgrowth Atria (right and left) far apart Bulbis cordis Common umbilical artery Endocardial tubes fuse forming tubular heart Heart begins beating Left ventricle, right ventricle, conotruncus Pericardium Primary head vein Sinus venosus Tubular heart begins folding Umbilical arteries Umbilical veins (right and left) Optic primordia fill neuromere D2 Optic sulcus in forebrain Otic pits Otic plates Chiasmatic plate Mesencephalic flexure Neural crest: Trigeminal, facioacoustic, glossopharyngeal-vagal, and occipitospinal Neural tube Neuromeres D1 and D2 (in diencephalon) Pontine region identifiable near cranial nerves VII and VIII Segment D in rhombencephalon Some secondary neuromeres Telencephalon Body cavities Hyoid arch Mandibular arch and maxillary process Somites: 4 - 12 pairs (3 weeks, 2 days): Nephrogenic cord emerges (at 10 somites) (3 weeks, 3 days): Carnegie Stage 11 Primordial germ cells begin moving from umbilical vesicle to hindgut Face: Maxillary and mandibular processes (bilaterally) Cloacal membrane Mesonephric duct emerges from nephrogenic cord Nephric vesicles Cystic primordium Hepatic diverticulum Liver Membrane between future mouth and throat may begin to rupture Angiogenesis along surface of central nervous system Aortic sac Atrioventricular canal Capillary plexus begins forming around brain and spinal cord Conotruncus Conus cordis emerging from right ventricle Endocardium Heart contractions produce peristaltic blood flow Interventricular septum Primordium of myocardium Sinus venosus separating from left atria Trabeculated outpouches along primary cardiac tube representing primordia of left and right ventricles Trigeminal and otic arteries Optic evagination (starting at 14 somites) Optic neural crest Optic vesicles form (17 to 19 somites) Otic vesicle Facio-vestibulocochlear ganglia Glossopharyngeal and vagal ganglia Lamina terminalis Mesencephalon contains tectum and tegmentum Neural crest production and migration continue Neurohypophysial primordia Neuropore (near brain) closes Notochord Trigeminal ganglia Segmentation of mesoblast alongside neural tube bilaterally Somites: 13 to 20 pairs (3 weeks, 3 days - 5 weeks, 6 days): All eight rhombomeres (Rh 1 through Rh 7, Rh D) (3 weeks, 5 days): Carnegie Stage 12 Telopharyngeal bodies Alimentary epithelium invades stroma of liver Alimentary epthelium proliferates in primordia of stomach, liver, and dorsal pancreas First part of pancreas Gastric portion of foregut elongates (25 to 28 somites) Hepatic primordium with abundant vascular plexus Omental bursa Oropharyngeal membrane is ruptured Pharyngeal arches 1, 2, and 3 Pharyngeal arches with dorsal and ventral parts Umbilical vesicle elongates Cervical sinus Laryngotracheal groove Lung bud Tracheo-esophageal septum Atrioventricular canal Common cardinal veins (right and left) Descending aorta Heart circulates blood to and from central nervous system, umbilical vesicle, and chorion Hepatocardiac channels (right and left) Internal carotid arteries Rostral and caudal cardinal veins along brain and spinal cord feeding common cardinal veins Septum primum and foramen primum sometimes present Sinu-atrial foramen prevents backflow into sinus venosus Sinus venosus collects veinous blood from entire embryo Unidirectional circulation Vitelline arteries and veins Otocyst nearly closed Nasal discs form part of ectodermal ring Brain involves 40% of neural tube Brain: Embryonic commissural plate Ectodermal ring complete Hypoglossal cord enters pharyngeal arch 4 Hypoglossal nucleus Lowermost spinal cord formation begins Mamillary recess Marginal layer in rhombencephalon Mesencephalic flexure at 90 degrees Mesencephalon with two neuromeres: M1 and M2 Motor neurons in basal plate of rhombencephalon Neural cord within caudal eminence Neural tube closes (lower back) Neurofibrils form in rhombencephalon Primary neurulation ends Primordia of ventral thalamus and subthalamus in diencephalon Sulcus limitans Sulcus limitans in midbrain Telencephalon (or telencephalic) medium Somites: 21 - 29 pairs Upper limb primordium at level of somites 8 to 10 Progressively C-shaped embryo (4 weeks): Carnegie Stage 13 Spleen primordia Thymic primordia Lower lip forms from merging of mandibular processes Melanoblasts in epidermis Periderm Gonadal ridge extends from C-7 to T-8 levels Primordial germ cells migrate to mesonephric ridges Primordial germ cells number several hundred Thyroid bilobed and attached to pharynx by thryoglossal duct Diaphragm primordia Glomeruli emerge in mesonephros Mesonephric duct attached to cloaca Nephric tubules now S-shaped Urogenital sinus Urorectal cleavage line Diverticulum ilei marks division between foregut and hindgut Hyoid arch sudivides into dorsal and ventral segments Intestines growing in length Opening between gut and umbilical vesicle decreases Pancreas: Ventral pancreas Pharyngeal pouches 1 through 4 Pharynx Pleuroperitoneal canals Stalk of umbilical vesicle lengthens and narrows Stomach assumes shape of a spindle Umbilical vesicle at height of development Vitelline duct Bronchial buds Embryonic stage begins Mesenchyme from coelomic epithelium surrounds esophagus and lung buds Right and left lung buds Trachea Anterior, middle, and posterior cerebral plexuses Aorta branches include dorsal intersegmental, lateral segmental, and ventral segmental arteries Aortic arches 4 and 6 Artery from the common iliac artery feeds each lower limb bud Atrioventricular bundle Cardiac contractions still under myogenic control Celiac artery, superior and inferior mesenteric arteries Circulatory system "well established" Common iliac arteries (right and left, from dorsal aorta bifurcation) Contractions well coordinated and sequential from sinus venosus to atria to ventricles Ductus venosus Functioning two-chamber heart Gas exchange through placenta begins Gelatinous reticulum (or cardiac mesenchyme) Heart chambers bulging with fluid Heart now functions as two parallel pumps Heart: Atrioventricular cushions (rostroventral and caudodorsal) Heart: Myocardium wall 3 to 4 cells thick Liver: Hepatocardiac vein Primary head veins (right and left) drain anterior, middle, and posterior cerebral plexuses and feed precardinal veins Septum primum, foramen primum Small arteries emerging throughout mesoderm Ventricle walls trabeculated Vertebral arteries Vitelline veins empty exclusively into hepatic plexus Otic invagination Retinal and lens discs Trigeminal, glossopharyngeal, and vagal preganglia Adenohypophysial pouch Brain: Commissural plate Cerebellum Common afferent tract Fourth ventricle Interstitial nucleus (part of medial longitudinal fasciculus) Isthmus rhombencephali (a new neuromere) Most cranial nerve ganglia Oculomotor and trochlear nuclei Terminal-vomeronasal neural crest Amnion surrounds connecting stalk and vitelline stalk Limb buds - the first sign of arms and legs Lower limb buds Umbilical cord emerging Upper and lower limb buds Less Events...

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	Unit 6: 5 to 6 Weeks
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(5 weeks, 2 days): Carnegie Stage 16 Apical epidermal ridges Mammary ridge Maxillary and premaxillary fields still widely separated Nipples emerge from mammary crest Gonad region separates from mesonephros Gonadal primordium Labioscrotal swelling Urogenital fold and groove Urorectal septum Suprarenal gland: Cortex primordium Suprarenal gland: Medulla Thyroid detaches from pharynx Thyroid with right and left lobes connected by an isthmus Cartilage in mandibular arch Hand area with central carpal region and digital plate with marginal vein Pre-chondrocranium: Otic capsule, nasal capsule, and parachordal condensations Primordia of primary palate Ribs: Primordia now present for all 12 pairs Vertebral column with 36 levels of ganglia and myotomes Extraocular premuscle masses Extraocular premuscle masses receive cranial nerves III, IV, or VI Gluteal mesoderm Infrahyoid premuscle masses Limb mesoderm Sternocleidomastoid-trapezius premuscle mass with spinal accessory nerve (CN11) Thigh and thigh mesoderm Tongue premuscle mass Metanephros at level of sacrum Urethral plate Falciform ligament Lesser omentum (ventral mesogastrun) Mesentery from end of duodenum to proximal half of colon Peritoneal cavity Rectum Stomach: Greater and lesser curvatures Yolk stalk disappears Bronchial tree expanding Cervical sinus diminished in size Epiglottis Primitive Larynx Anterior, middle, and posterior cerebral arteries Atrioventricular (AV) node Atrioventricular cushions fuse with interventricular septum Circle of Willis almost complete Conotruncal septum Endocardial cushions (rostroventral and caudodorsal) begin fusing around atrioventricular canal forming right and left atrioventricular canals and two separate blood streams External carotid artery Foramen primum disappearing Hepatic portal vein Infundibulum Jugular lymph sac Lateral atrioventricular cushions Mesencephalic artery Myelencephalic artery Perilental blood vessels Primitive renal plexus Right ventricle feeds sixth (pulmonary) aortic arches; left ventricle feeds fourth aortic arches Semilunar valves (aortic and pulmonic) are forming Ventricles each with three parts: inlet, trabecular pouch, and outflow tract Ventricles enlarge and deepen side-by-side forming an ever growing interventricular septum Auricular hillocks on hyoid arch named antitragus and helix Auricular hillocks on mandibular arch named tragus and crus Auricular hillocks precursors on hyoid arch Femoral and obturator nerves innervate rostrolateral part of lower limb Intercostal nerves Musculocutaneous, radial, ulna, and median nerves enter upper limb bud Nasal fin Nasal pits face more ventrally, still widely separated Nasofrontal groove Olfactory fibers connect nasal pits with brain Olfactory fibers enter brain Olfactory tubercle present Peroneal and tibial nerves innervate caudomedial part of lower limb Pigment in retina visible externally Primordium of cochlear pouch Tibial nerve innervates foot area Alar lamina emerging with dense rhombic lip All cranial nerves identifiable Archipallium, paleopallium, and neopallium Area epithelialis Brain: Primordial plexiform layer in area of future temporal lobe Cajal-Retzius cells Celiac plexus Cochlear nerve present Commissure of the trochlear nerve Diencephalic subthalamic nucleus Dorsal and ventral primary rami Dorsal and ventral thalami separated by groove Dorsal funiculus fibers reach medulla oblongata Epiphysis cerebri Glial cells identifiable adjacent to neurons Greater petrosal nerve Hypoglossal nerve reaches tongue Infundibular recess and infundibulum Interventricular foramen large Lumbar and sacral plexuses Marginal ridge Medial and lateral ridges of corpus striatum are continuous Median forebrain bundle Neurohypophysial outgrowth Olfactory tubercle Phrenic nerve Pontine flexure deepens Posterior commissure Recurrent laryngeal nerve Reticular formation more defined Retinal fissure closes Splanchnic nerve Sulcus limitans hippocampi Superior laryngeal nerve Sympathetic nervous system forms trunk Frontonasal prominence Second pharyngeal arch more prominent Third pharyngeal arch recedes (5½ weeks): Initial tooth formation (5½ weeks - 6 weeks): Subtle movement begins (5 weeks, 4 days - 6 weeks): Mitral and tricuspid valves (5 weeks, 5 days - 7 weeks, 1 day): Melanocytes in epidermis (5 weeks, 6 days): Carnegie Stage 17 Facial growth centers grow and begin merging forming nose and upper jaw Genital eminence forms phallus or genital tubercle Gonad grows into oval shape with irregular surface Cartilage in occipital sclerotomes (1-4) Digital rays in hand plate Femur: Chondrification begins Foot with rounded digital plate Hypoglossal foramen through sclerotome 4 Odontogenic epithelium ermerges in six areas (four maxillary and two mandibular) Primary palate components (right and left) fuse in midline Primitive palatine groove Vertebral centra begin chondrification Primordia of orbital muscles Calices Mesonephros can produce urine Pelvis of the ureter with three main divisions Vesico-urethral canal Appendix Biliary ducts within liver Dorsal and ventral pancreas fuse but retain separate ducts Duodenum enlarges proximal to and distal to bile and pancreatic ducts Esophagus developing a submucous coat surrounding epithelium Intestinal loop begins umbilical herniation Stomach regions include gastric canal, fundus, corpus (or body), and pyloric antrum Precursors of tracheal cartilages Anterior choroid artery Condensing mesenchyme around junction between left and right atria and cardiac tube is precursor to mitral and tricuspid valves Outflow tract rotates counterclockwise Right and left atrioventricular canals totally separated Auditory ossicles identifiable in mesenchyme Auricular hillocks begin to merge forming external ear Auricular hillocks with all components present Cell islands in olfactory tubercle Crescentic lens cavity Globular process emerges from each medial nasal process Hyomandibular groove enlarges (onset of concha and external auditory meatus formation) Nasal fin connecting nasal disc and surface epithelium Nasofrontal grooves Olfactory tubercle with cellular islands Primary lens fiber All 39 somite pairs present All parasympathetic cranial nerve ganglia identifiable All spinal nerves present Capillaries between adenohypophysis and hypothalamus Commissure of the oculomotor nerves Cortical nucleus in amygdaloid body Dentate and isthmic nuclei in cerebellum Dura begins forming in basal area Epiphysis cerebri with intermediate layer First hint of septal nucleus Frontal and tempororal poles of cerebral hemispheres Geniculate ganglion sparate from vestibulocochlear nerve Gustatory fibers separate from common afferent tract Hemispheric stalk Intermediate layer in tectum mesencephali Interventricular foramen Mesencephalon with intermediate layer Spinal cord reaches caudal tip of body Subarachnoid space Synapses in spinal cord between interneurons and primary afferent neurons Ventral thalamus with intermediate layer (6 weeks): Blood forming in liver Milk lines Handplates develop subtle flattening Tooth buds (primary teeth) Intestines fill base of umbilical cord Crown-heel length 1.6 cm Less Events...

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	Unit 8: 7 to 8 Weeks
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(7 weeks, 1 day): Carnegie Stage 20 Facial processes no longer distinguishable Ovaries full of primitive oogonia, intermediate pregranulosa cells, and mesenchyme Testes with short straight tubules Upper limbs with slightly flexed elbows Central tendon of diaphragm Renal vesicles with S-shaped lumina Submandibular gland: solid epithelial ducts enlarge and begin to branch Adenohypophysis with new capillaries on rostral surface Scalp vascular plexus Cochlear duct tip growing horizontally Lens cavity completely filled Optic commissure Optic fibers extend to optic chiasma Brain: Inferior colliculus (in mesencephalon) Cerebral hemispheres expand beyond lamina terminalis Cerebral hemispheres extend over two-thirds of diencephalon Cerebral peduncles Cuneate and gracile decussating fibers Interpeduncular groove Medial septal nucleus Nigrostriatal fibers Nucleus of diagonal band Primordium of falx cerebri Sacrocaudal spinal cord formation (secondary neurulation) complete Septum verum (7 weeks, 1 day - 8 weeks): Stomach: Folds in stomach wall (7 weeks, 2 days): Arteries and veins of heart complete (7 weeks, 3 days): Carnegie Stage 21 Volar pads begin to emerge on fingertips Cartilaginous nasal septum Chondrocranium with dorsum sellae and hypophysial fossa Dens (of second cervical vertebrae) Wrists slightly flexed Submandibular gland: Solid ducts with definitive branches Anterior and posterior choroid arteries Left superior vena cava disappears (Stages 21-23) Scalp vascular plexus moving toward vertex Cornea: Substantia propria layer Fibers of optic nerve reach brain Eyelids growing rapidly Anterior and inferior horns of lateral ventricle C-shaped lateral ventricle Cerebral hemispheres cover 75% of diencephalon Cerebral hemispheres cover more than half of diencephalon Cortical plate within primordial plexiform layer Glial and neurilemmal (Schwann) cells within cranial nerves Global pallidus internus Insula Internal fiber layer of cerebellum Lateral olfactory tract Primordium of dentate nucleus Pyramidal cells in hippocampus Subthalamic nucleus proper, entopeduncular nucleus, and globus pallidus externus within subthalamus Sulcus transversus rhombencephali Ventral part of lateral geniculate body (7½ weeks): Fingertips thicken Plantar pads toes EKG pattern similar to adult (7 weeks, 5 days): Carnegie Stage 22 Endolymphatic, jugular, and hypoglossal foramina Hands can reach one another and fingers can overlap Optic foramen, foramen rotundum, internal acoustic foramen Osteoblasts emerge Large glomeruli present within metanephros Submandibular gland: secondary branching with lumen formation starting at oral end of duct Chordae tendineae (Stages 22 and 23) Intradural veins (sinuses) Scalp vascular plexus 75% of the way to the vertex Cochlear duct's second loop growing upward Optic nerve acquires a sheath Scleral condensation Tragus and antitragus taking shape Eyelids continue growing rapidly over the surface of the cornea Brain: Claustrum Brain: Cortical plate within cerebral hemispheres Brain: Internal capsule with connections to epithalamus, dorsal thalamus, and mesencephalon Brain: Putamen Cerebral hemispheres cover 75% of diencephalon Commissural plate thickens Cortical plate expanding rapidly Folds in roof of third ventricle Nerve fibers between neopallial subplate and internal capsule Spinothalamic tract Sulcus dorsalis Superior colliculus Thalamocortical fibers (8 weeks): Carnegie Stage 23 Ductus deferens Interstitial cells forming within testis Penis: Corpus spongiosum Testicular tubules Male embryos are making testosterone already! Acromion and head of humerus Anterior inferior iliac spine Costal cartilage Enamel organ Femur: Head and acetabular fossa Glenoid fossa Greater trochanter Inguinal ligament Joint development: Cavitation underway in hip, knee, and ankle (in some embryos) Joint development: Cavitation underway in shoulder, elbow, and wrist (in some embryos) Nucleus pulposus Ossification underway in scapula and distal phalanges in some embryos Pelvis: Obturator foramen Pubic symphysis Scapular spine and notch Skull: Ossification underway in some embryos Skulll: Foramen magnum (wide) Sternoclavicular joint and manubrium Superior and inferior pubic rami Thyroid, tracheal, and cricoid cartilages Ulna: Styloid process and olecranon Vertebrae cartilaginous (33 or 34 in number) Depressor anguli oris muscle Digastric tendon Esophagus: Longitudinal muscles Frenulum of tongue Gluteus medius and gluteus minimus muscles Iliacus muscles Mylohyoid and anterior digastric muscles Mylohyoid, sternohyoid, and infrahyoid muscles Obturator externus, gluteus maximus, and hamstring muscles Obturator internus muscles Orbicularis oculi muscles Posterior digastric muscle Psoas tendon Rectus capitus posterior and obliquus superior capitus muscles Rectus femoris muscle Rectus sheath with anterior and posterior lamina Temporal and lateral pterygoid muscles Zygomaticus major muscle Bladder: Muscularis layer Kidneys at level of first three lumbar vertebrae Metanephros: Numerous large glomeruli Metanephros: Secretory tubules elongating and becoming convoluted Sinusal tubercle Urethra Gastrolienal ligament Nerves reaching intestinal loop Submandibular gland: lumen present in terminal portions of duct Submandibular gland: Mesodermal sheath surrounds gland Unfused uvula and secondary palate Costodiaphragmatic recess of pleural cavity Lingula of left upper lobe Pseudoglandular stage begins Azygos vein Blood supply to the brain closely resembles adult pattern Hemiazygos veins Inferior epigastric artery Inferior vena cava valve at junction of right atrium Pericardial sac Scalp vascular plexus nearing vertex Submandibular glands: Angiogenesis begins around epithelial tree (ducts) Superior sagittal sinus Cochlear duct's 2.5 coils nearly complete Cranial nerve distribution mimics adult pattern Eye: Secondary vitrous body Lens: Secondary lens fibers emerging Optic tract reaches ventral portion of lateral geniculate body Retina: Eight layers present Retina: Four of the ten adult layers present Tympanic membrane Eyelids fusing laterally and medially "The rhombencephalon...presents striking resemblance to that of the newborn." Amygdala area Brain represents 43% of embryo Brain: Caudate nucleus and putamen within corpus striatum Cerebellar commissures Cerebellum with external germinal layer Cerebral hemispheres cover lateral portion of diencephalon Choroid plexus now lobular Cortical plate covers nearly all of neopallial surface Dura lines entire vertebral canal Fasciculus cuneatus and fasciculus gracilis form the decussation of the medial lemnisci Greater palatine nerve Grey and white matter Hippocampus reaches temporal pole Inferior and superior cerebellar peduncles Medulla oblongata Most cisterns present Principal nucleus of inferior olivary nuclei Pyramidal decussations Right- and left-handedness emerges Subarachnoid space Suprapineal recess Suprascapular nerve Vermis of cerebellum Crown-heel length 4.3 cm Less Events...

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	Unit 10: 9 to 10 Weeks
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(10 weeks): Palatine tonsils Three-layered epidermis Now, all the bones are getting harder Tooth buds (secondary teeth) Physiologic herniation ends Left brachiocephalic vein Commissure of the fornix Crown-heel length 7.5 cm (10 weeks - 12 weeks): Langerhans cells enter epidermis

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	Unit 12: 11 to 12 Weeks
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(12 weeks): Sebaceous glands Many different hormones are present in pituitary gland All facial muscles in final positions Bile production begins in liver Bowel movements Corpus callosum Crura cerebri Myelination in spinal cord Crown-heel length 12 cm Head circumference 10 cm

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	Unit 14: 4 to 5 Months
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(18 weeks): Apocrine sweat glands Sweat glands (19 weeks): Melanin production Sulci on surface of cerebral hemispheres (20 weeks): Peyer's patches Surfactant production (low levels) Crown-heel length 28 cm Head circumference 20 cm

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	Unit 16: 6 to 7 Months
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(25 weeks): Intestinal lining contains all adult cell types (26 weeks): Terminal sac stage begins (28 weeks): Crown-heel length 39.5 cm

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	Unit 18: 8 to 9 Months
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(36 weeks): Surfactant production accelerates Brain weight 300 grams Crown-heel length 48.5 cm

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Nervous	Cardiovascular	Muscular	Early Events
Special Senses	Respiratory	Skeletal	General
Blood & Immune	Gastrointestinal	Endocrine	Growth Parameters
Skin/Integument	Renal/Urinary	Reproductive	Movement