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): Embryonic Period Begins Carnegie Stage 1 Fertilization Zygote formation (Day 1): Activation of the genome (Day 1 - Day 2): Cleavage First cell division - 24 to 30 hours Early pregnancy factor (EPF) (Day 2): Carnegie Stage 2 Metabolic activation of the oocyte (Day 3): Compaction Morula (meaning mulberry) (Day 4): Carnegie Stage 3 Free-floating blastocyst Blastocyst Chest and back surfaces seen Dorsal & ventral surfaces Embryonic disc (Day 5): Hatching Inner cell mass (Day 6): Carnegie Stage 4 Blastocyst attaches to endometrium Embryo attaches to inside wall of mother's womb Implantation begins with attachment of blastocyst to uterine wall (1 week): Carnegie Stage 5a Amniotic cavity formation begins Extra-embryonic mesoblast Hypoblast & epiblast Placenta formation begins Solid trophoblast 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 Bilateral symmetry Cloacal membrane Prechordal plate Primitive groove Right & left sides, head end Rostral-caudal orientation (2 weeks, 2 days): Carnegie Stage 7 Allantoic diverticulum Amnion with two cell layers Notochordal process Secondary villi Allantoic diverticulum Primordial germ cells Erythroblasts in umbilical vesicle Erythroblasts in yolk sac Three types of blood-forming cells in yolk sac Three types of hemopoietic cells in umbilical vesicle (2 weeks, 3 days): Anlage of the thyroid Portion of the thyroid (2 weeks, 4 days): Carnegie Stage 8 Connecting stalk Primitive pit Neural ectoderm Neural groove and folds Neural plate Notochordal and neurenteric canals Notochordal plate Prechordal plate with 1 retinal field Uteroplacental circulation well established Foregut, midgut, and hindgut Brain is first organ to appear (2 weeks, 5 days): Prechordal plate with 2 retinal fields (2 weeks, 6 days): Carnegie Stage 9 Somites 1 - 3 pairs of somites Cephalic and caudal folds Primitve streak reaches neurenteric canal Caudal eminence Forebrain, midbrain, and hindbrain Hindbrain with four rhombomeres Isthmus rhomencephali demarcates midbrain and hindbrain Neural crest Neural groove deepens substantially Primary neuromeres Otic disc First pharyngeal pouch Stomodeum forming Blood vessels appear in yolk sac, body, and amnion Blood vessels emerge independently 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 Heart Paired pericardial cavities Paired tubular heart Septum transversum Foregut First somites Numerous blood islands in umbilical vesicle Numerous blood-forming locations in yolk sac (3 weeks): Blood cells Blood vessels Less Events...

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	Unit 5: 4 to 5 Weeks
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(4 weeks, 3 days): Early eyes (4 weeks, 3 days - 5 weeks): Germ cells migrate to gonads (4½ weeks): Spleen (4 weeks, 4 days): Carnegie Stage 14 Adenohypophysis All 16 secondary neuromeres Cerebellum with intermediate layer Cerebral hemispheres Di-telencephalic sulcus Dorsal and ventral thalami Dorsal funiculus Mamillary region Medial and lateral longitudinal fasciculi Median ventricular eminence Pontine flexure Preoptico-hypothalamo-tegmental tract Rhombic lip Sulcus medius Synencephalon Tegmentum Tentorium cerebelli, medial portion Torus hemisphericus (TH) Velum transversum Ventral longitudinal fasciculus Eyes located on sides of head Lens pit Optic cups Epicardium Sinu-atrial (SA) node Right and left main stem bronchi Ureteric buds (4 weeks, 5 days): Carnegie Stage 15 Adult lamina terminalis Amygdaloid area Brain with five main sections Cerebellar plate First axodendritic synapses in cervical spinal cord First nerve fibers Five secondary brain subdivisions Habenular nucleus Habenulo-interpeduncular tract Hypothalamic sulcus Hypothalamus Lateral ventricle Lateral ventricular eminence Locus caeruleus Longitudinal zones in diencephalon Mesencephalic tract of CN 5 Most cranial nerves seen Postoptic recess Sclerotomic material around notochord (rhombomere D level) Subthalamus Superior colliculi Superior medullary velum Supramamillary commissure Synapses among motor neurons in spinal cord Tectobulbar tract Tentorium Third ventricle Trigemino-cerebellar tract Lens vesicles beneath surface Nasal pits Optic chiasma Optic stalk Lateral and medial nasal processes bilaterally Blood vessels penetrate diencephalon Coronary arteries (terminal end) Posterior communicating arteries Cecum Vertebrae well defined (5 weeks): Pigmented retina Pacemaker cells Pacemaker cells in sinus venosus Arytenoid and epiglottal swellings Bronchial tree branching accelerates Lobar pattern mimics adult pattern Lungs have 3 right lobes & 2 left lobes T-shaped laryngeal inlet Circular muscles in esophagus Permanent kidneys Hand plates ACTH [adrenocorticotropin hormone] Growth hormone Pituitary gland Head is 1/3 of entire embryo Less Events...

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	Unit 7: 6 to 7 Weeks
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(6 weeks, 2 days): Carnegie Stage 18 Embryo with dorsal concavity Archistriatum Brainwaves Cerebrospinal fluid production begins Choroid folds in fourth ventricle Choroid plexuses in fourth and lateral ventricles Dentate nucleus in internal cerebellar swellings External cerebellar swellings contain flocculi Four amygdaloid nuclei Hippocampus reaches olfactory region Nucleus ambiguus of the vagus (CN10) Prosencephalic septum Red nucleus Substantia nigra Supraoptic commissure Two or more semicurcular ducts Vomeronasal organ. nerve, and ganglion Nasolacrimal duct Nerve fibers in retina Optic fibers Primordium of stapes surrounds stapedial artery Bucconasal membrane Origin of left coronary artery Septum secundum and foramen ovale Submandibular glands Hands polygon-shaped (6½ weeks): Bone formation in collar bone Hand movement Ossification in clavicle & jaws Ossification in mandible & maxilla Volar pads on palms (6 weeks, 5 days): Carnegie Stage 19 Choana Ganglion of nervus terminalis Globus pallidus externus in the diencephalon Intermediate layer in dorsal thalamus Lateral recesses in fourth ventricle Lemniscal decussation Medial accessory olivary nucleus Nuclei of forebrain septum Nucleus accumbens Nucleus accumbens Occipital pole of cerebral hemispheres Subcommissural organ Cartilaginous styloid process Cochlear nuclei Optic fibers enter chiasmatic plate Circle of Willis complete Circulus arteriosus (Circle of Willis) complete Origin of right coronary artery Beginnings of occipital and sphenoid bones Three semicircular ducts (6 weeks, 6 days): Membrane blocking rectum ruptures Feet polygon-shaped (7 weeks): Head rotates Leg movements 4-chambered heart nearly complete Heart rate peaks (165 to 170 beats/minute) Bronchopulmonary segments complete Hiccups Ovaries Testes begin to differentiate Foot plates notched Tendons attach muscle to bone Insulin in pancreas B lymphocytes in liver 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 (8 weeks, 1 day - 9 weeks): Anal canal patent (8½ weeks): Neurons synapse in cerebral cortex Neurons synapse in cerebral cortex (marginal zone) Eyelids completely fused Plantar pads toes (9 weeks): External capsule Olivary nucleus with five components Face, hands, and feet sense light touch Bends hip & knee if sole of foot touched Drinking fluid is becoming routine Sucking the thumb The young fetus now sighs, stretches, moves the head, opens the mouth, and moves the tongue Tongue movement Small intestine peristalsis Oogonia replicating in fetal ovary Uterus Thyroid gland weighs 2 grams (9 weeks - 10 weeks): Early vocal cords Larynx recanalizes My weight will rise more than 75% this week Less Events...

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	Unit 11: 10 to 11 Weeks
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(10½ weeks): Thickened pads begin regressing on hands and feet Volar and plantar pads regress (11 weeks): Auditory cells: inner & outer hair cells The face now makes complex expressions Nose & lips completely formed Intestines absorb water & glucose Small intestine lined with villi External genitalia clearly male or female Intermediate layer Thyroid gland weighs 12 grams Immunological competence Crown-heel length (11 weeks - 12 weeks): Weight increases by 60% this week Less Events...

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	Unit 13: 3 to 4 Months
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(13 weeks): Most of body sensitive to touch Tooth development Tooth development (dentine) Cilia lining airways Crown-heel length 15 cm (14 weeks): 4-lobed cerebral cortex Cerebellum resembles adult structure Light touch to mouth evokes turn toward stimulus Fat deposits in cheeks Gender-dependent jaw movement Jaw motion more frequent in females Crown-heel length 17 cm (15 weeks): Digestive enzymes Fat deposits in abdomen, back, kidneys & shoulders Glucagon in fetal bloodstream Stem cells arrive in bone marrow Crown-heel length 19.5 cm (16 weeks): Hormonal stress response to invasive procedures Mom may begin to feel the fetus moving Quickening Tooth enamel Bronchial tree nearly complete Canalicular stage begins Colon lined with villi Fat deposits upper & lower limbs 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 Sratum corneum (21 weeks - 22 weeks): Age of viability (22 weeks): Behavioral states Cornea structure (23 weeks): Brain weight 100 grams (24 weeks): Blink-startle response; females before males Crown-heel length 34.5 cm

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	Unit 17: 7 to 8 Months
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(30 weeks): 6-layered cerebral cortex Breathing motions 30% to 40% of the time Head circumference 30 cm (32 weeks): Memory - music preferences Alveoli Lower esophagus muscles functional Glomeruli formation complete Crown-heel length 45 cm (32 weeks - 36 weeks): Prenatal food affects newborn taste preferences

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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): Birth Fetus initiates labor Fetus initiates labor via estrogen release Spinal cord ends at third lumbar vertebrae Major circulatory changes Transition to air breathing at birth Brain weight 350 grams Crown-heel length 50 cm Head circumference 35 cm Heart beats 54 million times before birth This timeline has more than 3.1 million configurations Less Events...

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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

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	Unit 2: 1 to 2 Weeks
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(1 week, 1 day): Bilaminar embryonic disc HCG (human chorionic gonadotropin) Pregnancy hormone (hCG) in mother's blood (1 week, 2 days): Carnegie Stage 5b Cells in womb engorged with nutrients Chorion Early decidual reaction Exocoelomic membrane Isolated trophoblastic lacunae Primary umbilical vesicle Primary yolk sac Yolk sac Corpus luteum of pregnancy Embryonic disc 0.1 mm diameter (1 week, 4 days): Carnegie Stage 5c Longitudinal axis Trophoblastic vascular circle (1 week, 5 days): Implantation complete Embryonic disc diameter: 0.15 to 0.20 mm (1 week, 6 days): Carnegie Stage 6 Amnion with single cell layer Chorionic villi Primitive streak Primitive streak and node Secondary umbilical vesicle Secondary yolk sac Angiogenesis in chorionic mesoblast Blood islands in umbilical vesicle Blood vessels in villi Less Events...

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	Unit 4: 3 to 4 Weeks
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(3 weeks, 1 day): Carnegie Stage 10 4 to 12 pairs of somites Body cavities Folding of the embryo begins Chiasmatic plate Forebrain with neuromeres D1 and D2 Mesencephalic flexure Neural tube Segment D in rhombencephalon Some secondary neuromeres Telencephalon Optic primordia Optic primordia fill neuromere D2 Optic sulcus in forebrain Otic pits Otic plates Hyoid arch Mandibular arch and maxillary process Oropharyngeal membrane Pharyngeal arches 1 and 2 Pharyngeal cleft 1 Second pharyngeal cleft and pouch Atria (right and left) far apart Circulatory system function begins Endocardial tubes fuse forming tubular heart Heart begins beating Left ventricle, right ventricle, conotruncus Pericardium Sinus venosus Tubular heart begins folding Umbilical arteries Umbilical veins Umbilical veins (right and left) Pharyngeal groove and ridge with laryngotracheal sulcus Respiratory outgrowth Respiratory primordium Hepatic plate Midgut emerging Thyroid primordium emerges from floor of pharynx (3 weeks, 2 days): Nephrogenic cord emerges (at 10 somites) (3 weeks, 3 days): Carnegie Stage 11 13 to 20 pairs of somites Segmentation of mesoblast alongside neural tube bilaterally Adenohypophysial primordia 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 Rostral neuropore closes Trigeminal ganglia Nasal plate Optic evagination (starting at 14 somites) Optic neural crest Optic sulcus Optic vesicle (17 to 19 somites) Optic vesicles form Otic vesicle Maxillary and mandibular processes (bilaterally) 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 Trigeminal and otic arteries Cystic primordium Hepatic diverticulum Liver Cloacal membrane Mesonephric duct emerges from nephrogenic cord Nephric vesicles Primordial germ cells begin moving from umbilical vesicle to hindgut Thyroid complete (3 weeks, 3 days - 5 weeks, 6 days): All eight rhombomeres (3 weeks, 5 days): Carnegie Stage 12 21 - 29 pairs of somites Ectodermal ring complete Progressively C-shaped embryo Brain involves 40% of neural tube Caudal neuropore closes Caudal neuropore closes Embryonic commissural plate 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 rhomencephalon Neural cord within caudal eminence Neural tube closes (lower back) Neurofibrils form in rhombencephalon Neuropore (lower back) closes Primary neurulation ends Primordia of ventral thalamus and subthalamus in diencephalon Sacrocaudal spinal cord formation (secondary neurulation) begins Sulcus limitans Sulcus limitans in midbrain Telencephalon medium Nasal discs form part of ectodermal ring Optic vesicles covered by sheath Otocyst nearly closed Oropharyngeal membrane is ruptured Pharyngeal arches 1, 2, and 3 Pharyngeal arches with dorsal and ventral parts Atrioventricular canal Common cardinal veins (right and left) Descending aorta Heart circulates blood to and from central nervous system, umbilical vesicle, and chorion Hepatic primordium with abundant vascular plexus Hepatocardiac channels (right and left) Internal carotid arteries Right and left dorsal aorta fuse forming descending aorta 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 Cervical sinus Laryngotracheal groove Lung bud Tracheo-esophageal septum 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) Omental bursa Umbilical vesicle elongates Earliest hint of upper limbs Upper limb primordium at level of somites 8 to 10 Telopharyngeal bodies (4 weeks): Carnegie Stage 13 Amnion surrounds connecting stalk and vitelline stalk Amnion surrounds embryo Umbilical cord Adenohypophysial pouch Cerebellum Commissural plate 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 Basement membrane of otic disc surrounds otic vesicle Endolymphatic appendage Otic invagination Otic vesicle closed Retinal and lens discs Lower lip forms from merging of mandibular processes Aortic arches 4 and 6 Atrioventricular bundle Atrioventricular cushions (rostroventral and caudodorsal) Basilar and vertebral arteries Cardiac contractions still under myogenic control Celiac artery, superior and inferior mesenteric arteries Circulatory system "well established" 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 rate (about) 113 beats/min Heart with right and left ventricles Hepatocardiac vein Myocardium wall 3 to 4 cells thick Septum primum, foramen primum Small arteries emerging throughout mesoderm Ventricle walls trabeculated Vitelline veins empty exclusively into hepatic plexus Bronchial buds Embryonic stage begins Lungs begin filling chest cavity Right and left lung buds Trachea Trachea or windpipe Windpipe Diverticulum ilei marks division between foregut and hindgut Esophagus and stomach Intestines growing in length Opening between gut and umbilical vesicle decreases Pharynx Pleuroperitoneal canals Small & large intestines Stalk of umbilical vesicle lengthens and narrows Stomach assumes shape of a spindle Umbilical vesicle at height of development Ventral pancreas Vitelline duct Glomeruli emerge in mesonephros Mesonephric duct attached to cloaca Nephric tubules now S-shaped Primordial germ cells migrate to mesonephric ridges Primordial germ cells number several hundred Urorectal cleavage line Diaphragm primordia Upper & lower limb buds Upper and lower limb buds Melanoblasts in epidermis Periderm Skin begins to thicken slowly Skin transparent Thyroid bilobed and attached to pharynx by thryoglossal duct Spleen primordia Thymic primordia Less Events...

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	Unit 6: 5 to 6 Weeks
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(5 weeks, 1 day): Wrists (5 weeks, 2 days): Carnegie Stage 16 All cranial nerves identifiable Archipallium, paleopallium, and neopallium Cochlear nerve present Commissure of the trochlear nerve Diencephalic subthalamic nucleus Epiphysis cerebri Glial cells identifiable adjacent to neurons Hypoglossal nerve reaches tongue Infundibular recess and infundibulum Marginal ridge Neurohypophysial outgrowth Olfactory tubercle Pontine flexure deepens Primordial plexiform layer in area of future temporal lobe Retinal fissure closes Olfactory fibers enter brain Olfactory tubercle present Frontonasal prominence Aortic and pulmonary valves Atrioventricular (AV) node Circle of Willis almost complete Epiglottis Thyroid detaches from pharynx Thyroid with right and left lobes connected by an isthmus (5½ weeks): Dental lamina Initial tooth formation Cartilage formation (5½ weeks - 6 weeks): Subtle movement begins (5 weeks, 4 days - 6 weeks): Mitral and tricuspid valves (5 weeks, 5 days): Nerve cells differentiating (5 weeks, 5 days - 7 weeks, 1 day): Melanocytes in epidermis (5 weeks, 6 days): Carnegie Stage 17 All 39 somite pairs present 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 First hint of septal nucleus Frontal and tempororal poles of cerebral hemispheres Gustatory fibers separate from common afferent tract Hemispheric stalk Intermediate layer in tectum mesencephali Interventricular foramen Spinal cord reaches caudal tip of body Subarachnoid space Synapses in spinal cord between interneurons and primary afferent neurons Blind nasal sac Cell islands in olfactory tubercle Crescentic lens cavity Nostril becomes continuous with nasal sac Primary lens fiber Primordia of orbital muscles Globular process emerges from each medial nasal process Anterior choroid artery Appendix Cartilage in occipital sclerotomes (1-4) Hypoglossal foramen through sclerotome 4 (6 weeks): Cerebral cortex Synapses form in spinal cord External ear Face withdraws from light touch around mouth Tooth buds (Deciduous) Tooth buds (primary teeth) Inferior vena cava Internal and external jugular veins Portal vein Diaphragm largely formed Intestines fill base of umbilical cord Handplates develop subtle flattening Joints Milk lines Nipples along side of trunk Adrenal glands Glucagon in pancreas Parathyroid glands Blood formation & lymphocytes in liver Blood forming in liver 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 Cerebral hemispheres expand beyond lamina terminalis Cerebral peduncles Cuneate and gracile decussating fibers Interpeduncular groove Lowermost spinal cord formation complete Medial septal nucleus Nigrostriatal fibers Nucleus of diagonal band Sacrocaudal spinal cord formation (secondary neurulation) complete Septum verum Optic commissure Facial processes no longer distinguishable (7 weeks, 1 day - 8 weeks): Folds in stomach wall (7 weeks, 2 days): Arteries and veins of heart complete Coronary arteries and veins complete (7 weeks, 3 days): Carnegie Stage 21 Anterior and inferior horns of lateral ventricle C-shaped lateral ventricle 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 Hippocampus Internal fiber layer of cerebellum Lateral olfactory tract Pineal recess Primordium of dentate nucleus Pyrimadal cells in hippocampus Subthalamic nucleus proper, entopeduncular nucleus, and globus pallidus externus within subthalamus Sulcus transversus rhombencephali Ventral part of lateral geniculate body Eyelids grow rapidly Cartilaginous nasal septum Anterior and middle cerebral arteries Anterior and posterior choroid arteries Chondrocranium with dorsum sellae and hypophysial fossa Dens (of second cervical vertebrae) (7½ weeks): Hands begin to touch face EKG pattern similar to adult Hands & feet touch in midline Knee joints Fingertips thicken Plantar pads toes Volar pads on fingertips (7 weeks, 4 days): Fingers completely separate (7 weeks, 5 days): Carnegie Stage 22 Cerebral hemispheres cover 75% of diencephalon Claustrum Commissural plate thickens Cortical plate expanding rapidly Folds in roof of third ventricle Internal capsule Internal capsule with connections to epithalamus, dorsal thalamus, and mesencephalon Nerve fibers between neopallial subplate and internal capsule Putamen Spinothalamic tract Thalamocortical fibers Intradural veins (sinuses) Endolymphatic, jugular, and hypoglossal foramina Optic foramen, foramen rotundum, internal acoustic foramen (7 weeks, 6 days): Toes free (8 weeks): Carnegie Stage 23, Embryonic Period Ends Brain highly complex Brain represents 43% of embryo Caudate nucleus and putamen within corpus striatum Cerebellar commissures Cerebellum with external germinal layer Choroid plexus now lobular Cortical plate covers nearly all of neopallial surface Cranial nerve distribution resembles adult pattern Dura lines entire vertebral canal Fasciculus cuneatus and fasciculus gracilis form the decussation of the medial lemnisci Handedness Hippocampus reaches temporal pole Inferior and superior cerebellar peduncles Insula Most cisterns present Pons Principal nucleus of inferior olivary nuclei Pyramidal decussations Subarachnoid space Suprapineal recess Optic tract reaches ventral portion of lateral geniculate body Retina with 4 of the10 layers found in adult Complex response to touch More frequent hand-to-face contact Squinting The embryo floats and rolls over in the womb Mouth opens & closes Blood supply to the brain closely resembles adult pattern Superior sagittal sinus Superior vena cava Breathing motion Diaphragm complete Pseudoglandular stage begins Longitudinal muscles in esophagus Peristalsis in large intestine Urine production and release Foramen magnum (wide) Joints resemble adult joints Hair in eyebrows & around mouth Skin multi-layered, loses transparency Hypothalamus Leydig cells (testes) produce testosterone Testes release testosterone Crown-heel length 4.3 cm Embryo contains approximately 1 billion (10^9) cells Over 4,000 body structures Less Events...

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	Unit 10: 9 to 10 Weeks
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(9½ weeks): I yawn when I want (9 weeks, 4 days): Yawns (10 weeks): Commissure of the fornix Corpus callosum begins Eyes roll downward reflexively Palatine tonsils Tonsils Tooth buds (permanent teeth) Tooth buds (secondary teeth) Left brachiocephalic vein Physiologic herniation ends Glomeruli formation begins Bone formation underway in most bones Ossification underway in most bones Epidermis with periderm, intermediate, and basal layers Fingernails & toenails begin Skin with three layers Three-layered epidermis Unique fingerprints Crown-heel length 7.5 cm (10 weeks - 12 weeks): Langerhans cells enter epidermis Less Events...

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	Unit 12: 11 to 12 Weeks
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(12 weeks): Corpus callosum Crura cerebri Myelination in spinal cord Taste buds throughout mouth Hands touch the mouth up to 50 times per hour All facial muscles in final positions Palate fuses Bile production begins in liver Bowel movements Bladder resembles smooth muscle Upper limbs reach final proportion Sebaceous glands Thyroid gland produces hormone T lymphocytes leave thymus Crown-heel length 12 cm Head circumference 10 cm Less Events...

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	Unit 14: 4 to 5 Months
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(17 weeks): Cerebral cortex activity Retina has discrete layers (18 weeks): Corpus callosum complete Speaking motion of larynx Apocrine sweat glands Cream-like substance protects skin Eccrine sweat glands sweat glands Vernix caseosa covers fetus Insulin secretion (19 weeks): Circadian rhythms Sulci on surface of cerebral hemispheres Number of oogonia peak within fetal ovaries Melanin production (20 weeks): Cochlea reaches adult size Organ of hearing reaches adult size Surfactant production (low levels) Peyer's patches All skin layers and structures Crown-heel length 28 cm Head circumference 20 cm (20 weeks - 24 weeks): Eyelids separate, eyes open and close Less Events...

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	Unit 16: 6 to 7 Months
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(25 weeks): Responds to taste Rods & cones Intestinal lining contains all adult cell types (26 weeks): Sense of smell Tear production Terminal sac stage begins Additional fat deposits decrease wrinkles (26 weeks - 38 weeks): Brain weight increases 400% to 500% (27 weeks): Pupils react to light (28 weeks): Distinguishes sounds of different frequencies Crown-heel length 39.5 cm Less Events...

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	Unit 18: 8 to 9 Months
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(34 weeks): Rapid weight gain (35 weeks): Amniotic fluid volume peaks Firm grip (36 weeks): Surfactant production accelerates Brain weight 300 grams Crown-heel length 48.5 cm

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Nervous	Cardiovascular	Musculoskeletal	Early Events
Special Senses	Respiratory	Skin	Growth Parameters
Movement	Gastrointestinal	Endocrine	General
Facial	Genitourinary	Blood & Immune	"Gee Whiz" Facts