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

Interactive Prenatal Development Timeline

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0-1 1-2 2-3 3-4 4-5 5-6 6-7 7-8 8-9 9-10 10-11 11-12 12-16 16-20 20-24 24-28 28-32 32-36 36-38

	Unit 1: The First Week
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(Day 0): Embryonic period begins Fertilization resulting in zygote formation (Day 1): Embryo is spherically shaped and called a morula comprised of 12 to 16 blastomeres (Day 2): Carnegie Stage 2: Cleaving Zygote Early pregnancy factor (EPF) Activation of the genome Blastomeres begin rapidly dividing (Day 3): Compaction (Day 4): Carnegie Stage 3: Free Blastocyst Embryonic disc Hypoblast & epiblast Inner cell mass See where the back and chest will be (Day 5): Hatching blastocyst (Day 6): Carnegie Stage 4: Attaching Blastocyst Embryo attaches to wall of uterus Solid synctiotrophoblast & cytotrophoblast (1 week): Chorion Chorionic cavity Extra-embryonic mesoderm (or mesoblast) Placenta begins to form Less Events...

	Unit 3: 2 to 3 Weeks
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(2 weeks, 1 day): 3 germ layers Cloacal membrane 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 Notochordal process Secondary villi (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 Caudal eminence 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 Oropharyngeal membrane Pharyngeal pouch 1 Stomodeum forming Blood vessels emerge simultaneously in umbilical vesicle, embryo proper, amnion, and connecting stalk Common umbilical artery Dorsal aortae (paired) First pair of aortic arches Heart: Cardiogenic plate, cardiac jelly, myocardial mantle, and endocardial plexus Left ventricle, right ventricle, conotruncus Paired pericardial cavities Paired tubular heart Hindbrain with four rhombomeres Isthmus rhombencephali demarcates midbrain and hindbrain Mesencephalon (or midbrain) Neural cord within caudal eminence Neural groove deepens substantially Primary neuromeres Three main divisions of brain Cephalic and caudal folds Neural crest: Rostral and facial Primitive streak reaches neurenteric canal Somites with central somitocoels: Pairs 1 through 3 Less Events...

	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 Common pulmonary vein drains pulmonary plexuses into left atrium Conotruncal ridges or cushions (remnants of cardiac jelly) Epicardium Left subclavian artery feeds left axillary artery, left vertebral artery, and and left thyrocervical trunk 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 Dorsal roots Hypoglossal nerve roots unite (CN XII) Lens and retina invaginate to form optic cup Primordium of cochlear duct Rami communicantes Spinal nerves reach muscle primordia Upper limb buds innervated Pigment in retina (external layer of optic cup) D1 and D2 no longer identifiable within diencephalon 75% of midbrain covered by marginal layer All 16 secondary neuromeres Brain enlarges 50% since Carnegie Stage 13 Brain: Cerebral hemispheres appear and begin rapid growth Brain: Lateral ventricles Cerebellum with intermediate and ventricular layers Cerebellum: Primordium found in alar plate of rhombomere 1 Corpora striata primordia connected by commissural plate Cranial nerve 3 Di-telencephalic sulcus Dorsal and ventral thalami Dorsal funiculus Hypothalamic sulcus Hypothalamus Mamillary region Medial and lateral longitudinal fasciculi Median ventricular eminence Pontine flexure Preoptic sulcus extends between optic evaginations Preoptico-hypothalamo-tegmental tract Primary meninx surrounds most of brain 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 Primordium of antitragus emerges from ventral subsegment of hyoid arch Gonad framework found in coelomic epithelium 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 Sclerotomic material around notochord (rhombomere D level) Vertebrae well defined Vertebral centra Primary urogenital sinus Ureteric bud extends to pelvis of the ureter Bladder and rectum are separating caudal to ureters Dense mesenchyme surrounds much of gastrointestinal tract Esophagus elongates, passes dorsal to carina and between main stem bronchi Gall bladder and cystic duct 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 Coronary arteries (terminal end) Foramen secundum begins in septum primum Left ventricle with thicker walls and greater volume than right Right subclavian artery originates from brachiocephalic artery and feeds right thyrocervical trunk and axillary and vertebral arteries Semilunar cusps Capsule present around lens Corneal epithelium overlying optic cup Ear: Endolymphatic duct Geniculate and vestibulocochlear ganglia separating Lens body now present containing some lens fibers Lower limb buds innervated Optic stalk Utricle, endolymphatic duct, and endolymphatic sac Utriculo-endolymphatic fold External ear primordia emerges from caudolateral portion of mandibular arch Face: Lateral and medial nasal processes bilaterally Lateral nasal processes along dorsolateral lip of nasal pits Optic chiasm 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 Habenular nucleus Habenulo-interpeduncular tract 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 Primordium of epiphysis Rhombomeres still identifiable 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 (CN IV) Hand plate emerges from distal upper limb bud Frontonasal prominence (5 weeks): Arytenoid and epiglottal swellings Lobar pattern mimics adult pattern T-shaped laryngeal inlet Pacemaker cells Less Events...

	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, radius, and ulna Humerus: Chondrocytes in phases one through three Scapula and clavicle Semicircular ducts form in order: anterior, posterior, and lateral Sternum: Episternal cartilage created from fusion of right and left sternal bars Tibia and fibula Toe rays sometimes present Deltoid muscle External and internal abdominal oblique muscles Levator scapulae muscle Longus cervicis and semispinalis cervicis muscles Pectoralis major muscles Platysma muscle Rectus abdominis muscle Rectus capitus posterior and semispinalis capitis muscles Serratus anterior muscles Splenius and longissimus muscles 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 Lingula of left upper lobe Aortic and pulmonary valves assuming shape of a cup Brachiocephalic veins, right and left Inferior vena cava Interventricular septum: membranous part begins forming Left coronary artery arises from aorta Mesenchyme ridges in place of future mitral and tricuspid valves 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) Bucconasal membrane Bucconasal membrane detaches opening up nasal airway Crus commune 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 tip emerges Nerve fibers in retina Optic fibers Retina's outer lamina heavily pigmented Vomeronasal nerve and ganglion Vomeronasal organ marked by groove and located in fold of lower medial nasal wall Ear: Stapes primordium surrounds stapedial artery External ear: Crus helicis forming from auricular hillocks two and three (from mandibular arch) Eyelid folds sometimes present Nasolacrimal duct begins as epithelial strand emanating from nasomaxillary groove Nostrils, nasal wings, and nasal septum easily seen Olfactory bulb sometimes with olfactory ventricle 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 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 Fourth ventricle: Choroid folds Hippocampus reaches olfactory region Interpeduncular fossa Neurohypophysis walls are folded Nucleus ambiguus of the vagus (CN10) Prosencephalic septum Red nucleus Substantia nigra Supra-optic 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) Intervertebral discs form from caudal condensed portion of sclerotomes Ischium and illium Labiodental lamina: Inner dental lamina and outer labiogingival band Laryngeal cartilages Limbs point forward (ventrally) Orbitosphenoid cartilage located within ectomeninx near optic stalk Ossification begins in maxilla (stages 19 -20) Primitive palate (or intermaxillary segment) Rib primordia become cartilaginous Ribs each have an identifiable head and shaft Trachea: Tracheal cartilage U-shaped labiodental lamina form along upper and lower oral cavity Vertebral column represented by cartilaginous centrum, neural arch, and short tranverse process Esophagus: Muscularis layer adjacent to esophageal plexus Gluteal muscle group Iliopsoas muscles Infrahyoid muscles Internal intercostal muscles Limb extensor muscles located dorsally Limb flexor muscles located ventrally Midgut: Muscularis Muscle tissue forming around phrenic nerve within septum transversum portion of diaphragm Pharyngeal constrictor muscle Premuscle mass of the muscles of mastication innervated by mandibular nerve Quadratus lumborum muscle Rhomboid and scalene muscles Sternocleidomastoid and trapezius muscles distinct and innervated by separate 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 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 rima oris replaces stomodeum Stomach wall layers: Mucosa, submucosa, muscularis, and serosa Submandibular and parotid gland buds Submandibular gland duct Bronchial tree: First generation of subsegmental bronchi complete 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 Circulus arteriosus (Circle of Willis) complete External iliac arteries Iliac lymph sac Intercostal and subcostal arteries Internal thoracic artery and costocervical trunk Mesenteric lymph sac Mesonephric artery feeds mesonephros, gonads, and suprarenal glands Papillary muscles Pontine, superior cerebellar, and anterior and posterior interior cerebellar arteries replace myelencephalic and metencephalic arteries Primitive marginal sinus drains diencephalon Primitive tentorial sinus drains cerebral vesical Primitive transverse and sigmoid sinuses Pulmonary arteries (right and left) Right coronary artery arises from aorta 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) Celiac, superior mesenteric, and inferior mesenteric preaortic ganglia Choana Cochlear duct tip grows upward 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 Glossopharyngeal nerve (CN IX) innervates stylopharyngeus premuscle mass Hypoglossal nerve (CN XII) innervates separating tongue muscles Linguogingival groove Nasolacrimal duct forms from maxillonasal groove Nasolacrimal ducts extend from medial eyes to primitive nasal cavity Nerve fibers begin extending from retina Optic fibers enter chiasmatic plate Primitive nasal cavity Primordial vitreous body 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 Eyelids: Upper and lower lids present and growing Adenohypophysis: Lateral lobes of pars tuberalis Adenohypophysis: Pars intermedia emerging Brain: Internal capsule formation underway Cerebral hemispheres cover half of diencephalon Dorsal and ventral cochlear nuclei Fourth ventricle: Lateral recesses Ganglion of nervus terminalis Globus pallidus externus in the diencephalon Habenular commissure Intermediate layer in dorsal thalamus 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 Zona limitans intrathalamica between dorsal and ventral thalami (6 weeks, 6 days): Cloacal membrane ruptures (7 weeks): Head rotates Ovaries 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...

	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...

	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

	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...

	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

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

	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!

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

1118 events on this timeline

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	Unit 2: 1 to 2 Weeks
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(1 week, 1 day): Amnioblasts present; amnion and amniotic cavity formation begins Bilaminar embryonic disc Positive pregnancy test (1 week, 2 days): Carnegie Stage 5b Corpus luteum of pregnancy Cells in womb engorged with nutrients Exocoelomic membrane Isolated trophoblastic lacunae Embryonic disc 0.1 mm diameter (1 week, 4 days): Carnegie Stage 5c Intercommunicating lacunae network Longitudinal axis Prechordal plate 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 Blood islands in umbilical vesicle 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 primitive node and Yolk sac Yolk sac Less Events...

	Unit 4: 3 to 4 Weeks
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(3 weeks, 1 day): Carnegie Stage 10 Thyroid primordium emerges from floor of pharynx Nephrogenic cord emerges (at 10 somites) Cloaca Common coelomic cavity divides into peritoneal, pericardial, and pleural cavities Liver: Hepatic plate (endoderm) Midgut emerging 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 Endocardial tubes fuse forming tubular heart Heart begins beating Pericardial sac Pericardium Primary head vein Sinus venosus Tubular heart begins folding Umbilical arteries Umbilical veins (right and left) Optic primordia fill neuromere D2 Chiasmatic plate Mesencephalic flexure Neural tube Neuromeres D1 and D2 (in diencephalon) Optic sulcus in forebrain Pontine region identifiable near cranial nerves VII and VIII Segment D in rhombencephalon Some secondary neuromeres Superior colliculus Telencephalon Telencephalon (or telencephalic) medium Body cavities Hyoid arch Mandibular arch and maxillary process Neural crest: Trigeminal, facioacoustic, glossopharyngeal-vagal, and occipitospinal Somites: Pairs 4 through 12 (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 Internal carotid arteries 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 Facio-vestibulocochlear ganglia (CN VII, CN VIII) Glossopharyngeal and vagal ganglia Optic evagination (starting at 14 somites) Otic vesicle Trigeminal ganglia (CN V) Adenohypophysial pouch Adenohypophysis Lamina terminalis Mesencephalon contains tectum and tegmentum Neural crest production and migration continue Neurohypophysial primordia Neuropore (near brain) closes Notochord Segmentation of mesoblast alongside neural tube bilaterally Somites: Pairs 13 through 20 (3 weeks, 3 days - 5 weeks, 6 days): All eight rhombomeres (Rh 1 through Rh 7, Rh D) - Present in stages 11 through 17 (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 arch 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) Rostral and caudal cardinal veins along brain and spinal cord feeding common cardinal veins Septum primum and foramen primum sometimes present Septum primum, foramen primum Sinu-atrial foramen prevents backflow into sinus venosus Sinus venosus collects veinous blood from entire embryo Superior vena cava, inferior vena cava, and sinus venosus collecting all venous blood Unidirectional circulation Vitelline arteries and veins Hypoglossal cord (CN XII) enters pharyngeal arch 4 Otocyst nearly closed Nasal discs form part of ectodermal ring Brain involves 40% of neural tube Brain: Embryonic commissural plate Ectodermal ring complete Hypoglossal nucleus (CN XII) 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 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 Somites: Pairs 21 through 29 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 Gonadal ridge extends from C-7 to T-8 levels Primordial germ cells migrate to mesonephric ridges Primordial germ cells number several hundred Urorectal septum 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 Intestines growing in length Mesentery from end of duodenum to proximal half of colon 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 Mesenchyme from coelomic epithelium surrounds esophagus and 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 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 Primary head veins (right and left) drain anterior, middle, and posterior cerebral plexuses and feed precardinal veins Small arteries emerging throughout mesoderm Ventricle walls trabeculated Vertebral arteries Vitelline veins empty exclusively into hepatic plexus Most cranial nerve ganglia Trigeminal, glossopharyngeal, and vagal preganglia Otic invagination Terminal-vomeronasal neural crest Brain: Commissural plate Cerebellum Common afferent tract Fourth ventricle Interstitial nucleus (part of medial longitudinal fasciculus) Isthmus rhombencephali (a new neuromere) Oculomotor (CN III) and trochlear nuclei (CN IV) in mesencephalon (midbrain) and isthmus respectively Retinal and lens discs Amnion surrounds connecting stalk and vitelline stalk Hyoid arch sudivides into dorsal and ventral segments Limb buds - the first sign of arms and legs Lower limb buds Umbilical cord emerging Upper and lower limb buds Less Events...

	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 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 Extra-ocular premuscle masses receive cranial nerve fibers [oculomotor (CN III), trochlear (CN IV), and abducens (CN VI) nerves] 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 Lesser omentum (ventral mesogastrun) 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 of right ventricle Jugular lymph sac Lateral atrioventricular cushions Mesencephalic artery Myelencephalic artery Perilental blood vessels Primitive cavernous sinus drains primitive maxillary and supraorbital veins Primitive renal plexus Right ventricle feeds sixth (pulmonary) aortic arches; left ventricle feeds fourth aortic arches Semilunar valves (aortic and pulmonary) 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 Celiac plexus Cochlear nerve present Femoral and obturator nerves innervate rostrolateral part of lower limb Hypoglossal nerve (CN XII) reaches tongue Intercostal nerves Lumbar and sacral plexuses Musculocutaneous, radial, ulna, and median nerves enter upper limb bud 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 Phrenic nerve Pigment in retina visible externally Primordium of cochlear pouch Tibial nerve innervates foot area Auricular hillocks on hyoid arch (antitragus and helix) Auricular hillocks on mandibular arch (tragus and crus) 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 Commissure of the trochlear nerve Diencephalic subthalamic nucleus Dorsal and ventral thalami separated by groove Dorsal funiculus fibers reach medulla oblongata Epiphysis cerebri Glial cells identifiable adjacent to neurons Greater petrosal nerve Hippocampus: Gyrus dentatus Infundibular recess and infundibulum Interventricular foramen large Marginal ridge Medial and lateral ridges of corpus striatum are continuous Median forebrain bundle Neurohypophysial outgrowth Olfactory tubercle Pontine flexure deepens Posterior commissure Recurrent laryngeal nerve Reticular formation more defined Retinal fissure closes Splanchnic nerve Sulcus limitans hippocampi Superior laryngeal nerve Second pharyngeal arch more prominent Third pharyngeal arch recedes (5½ weeks): Initial tooth formation (5½ weeks - 6 weeks): Subtle movement begins (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 Auditory ossicles identifiable in mesenchyme Cartilage in occipital sclerotomes (1-4) Digital rays in hand plate Femur: Chondrification begins Foot with rounded digital plate Hypoglossal foramen (or canal) through sclerotome 4 (area of future occipital bone) Odontogenic epithelium ermerges in six areas (four maxillary and two mandibular) Primary palate components (right and left) fuse in midline Primitive palatine groove Primordium of cartilage within nasal septum Vertebral centra begin chondrification Primordia of orbital muscles Calices Mesonephros can produce urine Pelvis of the ureter with three main divisions Vesico-urethral canal 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 Primordial vermiform appendix Stomach regions include gastric canal, fundus, corpus (or body), and pyloric antrum Trachea: Precursors of tracheal cartilages 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 All parasympathetic cranial nerve ganglia identifiable All spinal nerves present Cell islands in olfactory tubercle Crescentic lens cavity Geniculate ganglion separate from vestibulocochlear nerve Globular process emerges from each medial nasal process Nasal fin connecting nasal disc and surface epithelium Nasofrontal grooves Olfactory tubercle with cellular islands 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 temporal poles of cerebral hemispheres Gustatory fibers separate from common afferent tract Hemispheric stalk Intermediate layer in tectum mesencephali Interventricular foramen Mesencephalon with intermediate layer Somites: Pairs 38 and 39 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 Medial skull cartilages: Parachordal, hypophyseal, and trabecular Tooth buds (primary teeth) Intestines fill base of umbilical cord Crown-heel length 1.6 cm Less Events...

	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 Diaphragm: Central tendon 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 Interpeduncular groove Medial septal nucleus Nigrostriatal fibers Nucleus of diagonal band Sacrocaudal spinal cord formation (secondary neurulation) complete Sensory pathways: Cuneate and gracile decussating fibers Septum verum Spinothalamic tract (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 Chondrocranium with dorsum sellae and hypophysial fossa Dens (of second cervical vertebrae) Sternoclavicular joint and manubrium Trachea: Thyroid cartilage Wrists slightly flexed Gluteus medius and gluteus minimus muscles Iliacus muscles Mylohyoid and infrahyoid muscles Orbicularis oculi muscles 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 Brain: Insula within cerebral hemisphere 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 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): Endolymphatic and jugular foramina Hands can reach one another and fingers can overlap Optic foramen, foramen rotundum, internal acoustic foramen Osteoblasts emerge Pelvis: Obturator foramen Obturator internus muscles Rectus femoris muscle Large glomeruli present within metanephros Submandibular gland: Secondary branching with lumen formation starting at oral end of duct Costodiaphragmatic recess of pleural cavity 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 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 Thalamocortical fibers (8 weeks): Carnegie Stage 23 Ductus deferens Interstitial cells forming within testis Testicular tubules Male embryos are making testosterone already! Anterior inferior iliac spine Costal cartilage Enamel organ Femur: Head and acetabular fossa Glenoid fossa Greater trochanter Head of humerus 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 (from notochord) Ossification underway in scapula and distal phalanges in some embryos Pubic symphysis Scapular spine and notch Skull: Foramen magnum (wide) Skull: Ossification underway in some embryos Superior and inferior pubic rami Ulna: Styloid process and olecranon Vertebrae cartilaginous (33 or 34 in number) Anterior digastric muscles Depressor anguli oris muscle Esophagus: Longitudinal muscles Obliquus superior capitus muscle Obturator externus, gluteus maximus, and hamstring muscles Posterior belly of the digastric muscle Psoas tendon Rectus sheath with anterior and posterior lamina Temporal and lateral pterygoid muscles Zygomaticus major muscle 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 (edge of unfused palatine shelf) and secondary palate 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 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 Retina: Eight layers present Retina: Four of the ten adult layers present Tympanic membrane Eyelids fusing laterally and medially Optic tract reaches ventral portion of lateral geniculate body "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 Most cisterns present Principal nucleus of inferior olivary nuclei Pyramidal decussations Right- and left-handedness emerges Suprapineal recess Suprascapular nerve Vermis of cerebellum Crown-heel length 4.3 cm Embryonic Period Ends The 8-week embryo has formed more than 4,000 permanent body parts. Less Events...

	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 Commissure of the fornix Crown-heel length 7.5 cm (10 weeks - 12 weeks): Langerhans cells enter epidermis

	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 Bowel movements Liver: Bile production begins Corpus callosum Crura cerebri Myelination in spinal cord Crown-heel length 12 cm Head circumference 10 cm

	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

	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

	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

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