| Unit 1: The First Week | |
| 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 | – | – |  Early pregnancy factor (EPF) |
| | | Activation of the genome |
| | | Blastomeres begin rapidly dividing |
| Day 3 | – | – | Compaction |
| Day 4 | – | – | Embryonic disc |
| | | Hypoblast & epiblast |
| | | Inner cell mass |
| | | See where the back and chest will be |
| Day 5 | – | – | Hatching blastocyst |
| Day 6 | – | – | Embryo attaches to wall of uterus |
| | | Solid synctiotrophoblast & cytotrophoblast |
| 1 week | – | – | Chorion |
| | | Chorionic cavity |
| | | Extra-embryonic mesoderm (or mesoblast) |
| | | Placenta begins to form |
| Unit 2: 1 to 2 Weeks | |
| 1 week, 1 day | – | – | Amnioblasts present; amnion and amniotic cavity formation begins |
| | | Bilaminar embryonic disc |
| | | Positive pregnancy test |
| 1 week, 2 days | – | – | 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 | – | – | 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 | – | – |  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 |
| | | |
| Unit 3: 2 to 3 Weeks | |
| 2 weeks, 1 day | – | – | 3 germ layers |
| | | Cloacal membrane |
| | | Primitive groove |
| | | Rostral-caudal orientation |
| 2 weeks, 2 days | – | – | 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 | – | – |  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 | – | – | 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 |
| Unit 4: 3 to 4 Weeks | |
| 3 weeks, 1 day | – | – | 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 |
| | | Otic pits |
| | | 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 | – | – | 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) |
| | | Neural crest: Optic crest emerges during Carnegie Stages 11 and 12 |
| | | Nose: Nasal plate |
| | | Optic vesicles form (17 to 19 somites) |
| | | 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 | – | – | 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 |
| | | Optic vesicles covered by sheath (formed by mesencephalic and optic crest) |
| | | 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 | – | – | 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 |
| | | Basement membrane of otic disc surrounds otic vesicle |
| | | Endolymphatic appendage |
| | | Otic invagination |
| | | Otic vesicle closes |
| | | 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 |
| | | |
| Unit 5: 4 to 5 Weeks | |
| 4 weeks, 4 days | – | – | 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 |
| | | External ear: Auricular hillocks merging |
| | | Eyes located on sides of head |
| | | Lens pits |
| | | Lens vesicle open to surface (lens pore) |
| | | Nose: Nasal pits |
| | | Nose: Nasal plate (or disc) flat or concave |
| | | 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 | – | – | 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 |
| | | Lens vesicles closed, pores absent |
| | | Nose: Nasal discs recede forming 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 |
| | | |
| Unit 6: 5 to 6 Weeks | |
| 5 weeks, 2 days | – | – | 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) |
| | | Blind nasal sac |
| | | Nasal fin |
| | | 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 |
| 5� weeks | – | – | Initial tooth formation |
| 5� weeks - 6 weeks | – | – |  Subtle movement begins |
| 5 weeks, 5 days - 7 weeks, 1 day | – | – | Melanocytes in epidermis |
| 5 weeks, 6 days | – | – | 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 |
| | | Hyomandibular groove enlarges (onset of concha and external auditory meatus formation) |
| | | Medial rims of nasal pits form nasal septum |
| | | Nostril becomes continuous with nasal sac |
| | | Primary lens fibers |
| | | Retinal fissure closed |
| | | 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 |
| | |  Anterior choroid artery |
| 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 |
| | | |
| Unit 7: 6 to 7 Weeks | |
| 6 weeks, 2 days | – | – | 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 |
| | | Choanae |
| | | Conjunctival sac marked by groove |
| | | Cornea and conjunctiva |
| | | Ear: Stapes primordium surrounds stapedial artery |
| | | External ear: Crus helicis forming from auricular hillocks two and three (from mandibular arch) |
| | | Eyelid folds sometimes present |
| | | Nasal fin splits forming choanae and bucconasal membrane |
| | | Nasolacrimal duct begins as epithelial strand emanating from nasomaxillary groove |
| | | Nostrils, nasal wings, and nasal septum easily seen |
| | | Olfactory bulb sometimes with olfactory ventricle |
| | | Primary lens fibers filling lens vesicle cavity |
| | | 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 |
| 6� weeks | – | – | Volar pads on palms |
| 6 weeks, 5 days | – | – | 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 |
| | | Saccule and cochlear duct |
| | | 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 |
| | | |
| Unit 8: 7 to 8 Weeks | |
| 7 weeks, 1 day | – | – | 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 |
| | | Cornea with three layers |
| | | 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 | – | – | 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 |
| 7� 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 |
| | | Optic nerve acquires a sheath |
| | | 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 | – | – | 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. |
| | | |
| Unit 9: 8 to 9 Weeks | |
| 8 weeks, 1 day | – | – | Humerus: Bone marrow replaces cartilage |
| 8 weeks, 1 day - 9 weeks | – | – | Anal canal patent |
| 8� 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 |
| | | |
| Unit 10: 9 to 10 Weeks | |
| 9 weeks - 10 weeks | – | – | Larynx recanalizes |
| 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 |
| | | |
| Unit 11: 10 to 11 Weeks | |
| 10 weeks - 12 weeks | – | – | Langerhans cells enter epidermis |
| 10� weeks | – | – | Volar and plantar pads regress |
| 11 weeks | – | – | Intermediate layer |
| | | Intestines absorb water & glucose |
| | | Small intestine lined with villi |
| | | Crown-heel length |
| | | |
| Unit 12: 11 to 12 Weeks | |
| 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 13: 3 to 4 Months | |
| 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 |
| | | |
| Unit 14: 4 to 5 Months | |
| 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 15: 5 to 6 Months | |
| 21 weeks | – | – | Periderm disappears |
| | | Stratum corneum |
| 22 weeks | – | – | Cornea structure |
| | | Behavioral states |
| 23 weeks | – | – | Brain weight 100 grams |
| 24 weeks | – | – | Crown-heel length 34.5 cm |
| | | |
| Unit 16: 6 to 7 Months | |
| 25 weeks | – | – | Intestinal lining contains all adult cell types |
| 26 weeks | – | – | Terminal sac stage begins |
| 28 weeks | – | – | Crown-heel length 39.5 cm |
| Unit 17: 7 to 8 Months | |
| 30 weeks | – | – | Head circumference 30 cm |
| 32 weeks | – | – | Esophagus: Lower esophagus muscles functional |
| | | Crown-heel length 45 cm |
| Unit 18: 8 to 9 Months | |
| 36 weeks | – | – | Surfactant production accelerates |
| | | Brain weight 300 grams |
| | | Crown-heel length 48.5 cm |
| Unit 19: 9 Months to Birth | |
| 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! |
| |
| 66 weeks, 5 days | – | – | Premuscle cells form sheets representing muscles of facial expression |
Growth Parameters