Appendix 1. Correlated Human Development - Review of Medical Embryology Book

Age (weeks): 2.5

SIZE (CR in mm): 1.5

Body form: embryonic disk flat. Primitive streak prominent. Neural groove indicated
Mouth: not developed
Pharynx and derivatives: not developed
Digestive tube and glands: gut not as yet distinct from yolk sac
Respiratory system: not developed
Coelom and mesenteries: extraembryonic coelom present. Embryonic coelom almost ready to make its appearance
rogenital system: allantois present
ascular system: blood islands appear on chorion and yolk sac. Cardiogenic plate reversing
keletal system: head process for notochordal plate is present
Muscular system: not developed
Integumentary system: ectoderm seen as a single layer
Nervous system: neural groove is indicated
Sense organs: not developed

Age (weeks): 3.5

SIZE (CR in mm): 2.5

Body form: neural groove deepens and closes except at ends. Somites (1-16?) present. Cylindric body constricting from yolk sac. Branchial arches 1 and 2 seen
Mouth: mandibular arch is prominent. Stomodeum a definitive pit. Oral membrane ruptures
Pharynx and derivatives: pharynx broad and flat. Pharyngeal pouches forming. Thyroid gland makes its appearance
Digestive tube and glands: foregut and hindgut present. Yolk sac broadly attached at midgut. Liver bud is present. Cloaca and cloacal membrane are seen
Respiratory system: respiratory primordium appears as a groove on pharyngeal floor
Coelom and mesenteries: embryonic coelom a U-shaped canal with a large pericardial cavity. Septum transversum seen. Mesenteries forming at this time. Mesocardium begins to atrophy
Urogenital system: all pronephric tubules are formed. Pronephric duct growing caudally as a blind tube. Cloaca and cloacal membrane are present
Vascular system: primitive blood cells and vessels are present. Embryonic blood vessels form a paired symmetric system. The heart tubes fuse, bend in an S-shape, and the heartbeat begins
Skeletal system: mesodermal segments are appearing (1-16?). Older somites do not demonstrate sclerotomes. Notochord appears as a cellular rod
Muscular system: mesodermal segments appearing (1-16?). Older somites begin to demonstrate myotome plates
Integumentary system: no new developments
Nervous system: neural groove is prominent but rapidly closing. Neural crest is a continuous band
Sense organs: optic vesicle and auditory placode are seen. Acoustic ganglia appearing

Age (weeks): 4.0

SIZE (CR in mm): 5.0

Body form: branchial arches completed. Flexed heart is prominent. Yolk stalk is slender. All 40 somites are present. Limb buds appear. Otocyst and eye are evident. Body is now C-shaped and flexed
Mouth: maxillary and mandibular processes are prominent. Tongue primordia is present. Rathke's pouch becomes evident
Pharynx and derivatives: 5 pharyngeal pouches are present. Pouches 1-4 have closing plates. The primary tympanic cavity is indicated. The thyroid gland is now a stalked sac
Digestive tube and glands: esophagus is short. The stomach is spindle-shaped. Intestine is a simple tube. Liver cords, ducts, and gallbladder are forming. Both pancreatic buds appear. The cloaca is at its full development
Respiratory system: trachea and paired lung buds become prominent. Laryngeal opening is a simple slit
Coelom and mesenteries: coelom still a continuous system of cavities. Dorsal mesentery is a complete median structure. Omental bursa is now indicated
Urogenital system: pronephros has degenerated. Pronephric (mesonephric) duct reaches the cloaca. Mesonephric tubules are differentiating rapidly. Metanephric bud is seen pushing into secretory primordium
Vascular system: hematopoiesis is seen on the yolk sac. The paired aortae fuse. The aortic arches and cardinal veins are completed. The dilated heart shows a sinus, atrium, ventricle, and bulbus
Skeletal system: all 40 somites are present. Sclerotomes are massed as primitive vertebrae around the notochord
Muscular system: all 40 somites are present
Integumentary system: not remarkable
Nervous system: neural tube is closed. Three primary vesicles of brain are seen. Nerves and ganglia are forming. Ependymal, mantle, and marginal layers are seen
Sense organs: optic cup and lens pit are forming. Auditory pit becomes the closed detached otocyst. Olfactory placodes arise and differentiate nerve cells

Age (weeks): 5.0

SIZE (CR in mm): 8.0

Body form: nasal pits are present. Tail is prominent. Heart, liver, and mesonephros are prominent. Umbilical cord organizes
Mouth: jaws are outlined. Rathke's pouch is now a stalked sac
Pharynx and derivatives: pharyngeal pouches develop dorsal and ventral diverticulae. Thyroid becomes bilobed. Thyroglossal duct atrophies
Digestive tube and glands: tail-gut atrophies. Yolk stalk detaches. Intestine elongates into a loop. Cecum is now evident
Respiratory system: bronchial buds presage future lung lobes. Arytenoid cartilage swellings and epiglottis are indicated
Coelom and mesenteries: pleuropericardial and pleuroperitoneal membranes are forming. Ventral mesogastrium pulls away from the septum
Urogenital system: mesonephros reaches its caudal limit. Ureteric and pelvic primordia become distinct. Genital ridge bulges
Vascular system: primitive vessels extend into head and limbs. Vitelline and umbilical veins are transforming. Myocardium is condensing. Cardiac septa appearing. Spleen becomes evident
Skeletal system: condensations of'mesenchyme presage many future bones
Muscular system: premuscle niasses are seen in head, trunk, and limbs
Integumentary system: epidermis acquiring a second layer (periderm)
Nervous system: 5 brain vesicles are seen. Cerebral hemispheres bulging. Nerves and ganglia more clearly seen. Suprarenal cortex beginning to form
Sense organs: choroid fissure is prominent. Lens vesicle is free. Vitreous anlage is appearing. Otocyst elongates, and endolymphatic duct is budded off. Olfactory pit deepens

Age (weeks): 6.0

SIZE (CR in mm): 12.0

Body form: upper jaw components are prominent but still separate. Lower jaw halves fuse. Head becomes dominant in size. Cervical flexure is marked. External ear appears. Limbs become more clearly recognizable
Mouth: lingual primordia fusing. Foramen cecum is established. Labiodental laminae begin to appear. Parotid and submandibular gland buds appear
Pharynx and derivatives: thymic sacs, ultimobranchial sacs, and solid parathyroids are conspicuous and ready to detach. Thyroid gland becomes solid, converts into plates
Digestive tube and glands: stomach rotating, intestinal loop under torsion. Hepatic 10bes are identifiable. Cloaca is subdividing
Respiratory system: definitive pulmonary lobes are indicated. Bronchi are subbranching. Laryngeal cavity temporarily obliterated
Coelom and mesenteries: pleuropericardial communications close. Mesentery expands as the intestine forms a loop
Urogenital system: cloaca subdividing. Pelvic anlage sprouts pole tubules. Sexless gonads and genital tubercle are prominent. Mullerian duct appearing
Vascular system: hematopoiesis in the liver is seen. Aortic arches are transforming. Left umbilical vein and ductus venosus become important. Bulbus is absorbed into right ventricle. Heart acquires its general definitive shape
Skeletal system: first appearance of chondrification centers. Desmocranium seen
Muscular system: myotomes become fused into a continuous column and spread ventrally. Muscle segmentation generally lost
Integumentary system: milk line is now present
Nervous system: 3 primary flexures of brain are seen. Diencephalon is large. Nerve plexuses are present. Epiphysis recognizable. Sympathetic ganglia forming segmental masses. Meninges are beginning to appear
Sense organs: optic cup shows nervous and pigment layers. Lens vesicle thickens. Eyes are set at 160 degrees. Nasolacrimal duct seen. Modeling of external, middle, and internal ear is beginning. Vomeronasal organ seen

Age (weeks): 7.0

SIZE (CR in mm): 17.0

Body form: branchial arches are lost. Cervical sinus is obliterated. Face and neck are forming. Digits are evident. Back straightening. Heart and liver determine shape of body ventrally. Tail is now regressing
Mouth: lingual primordia merge into a single tongue. Separate labial and dental laminae are visible. Jaws are formed and begin to ossify. Palate folds are present and are separated by the tongue
Pharynx and derivatives: thymus becomes elongated and loses its lumen. Parathyroids become trabeculate and associate with the thyroid. Ultimobranchial bodies fuse with the thyroid. The thyroid becomes crescentic
Digestive tube and glands: stomach attaining final shape and position. Duodenum is temporarily occluded. Intestinal loops herniate into cord. Rectum separates from bladder-urethra. Anal membrane ruptures. Dorsal and ventral pancreatic primordia fuse
Respiratory system: larynx and epiglottis are well outlined with a T-shaped orifice. Laryngeal and tracheal cartilages are foreshadowed. Conchae begin to appear. Primary choanae are rupturing
Coelom and mesenteries: the pericardium is extended by splitting from body wall. Mesentery is expanding rapidly as the intestine coils. Ligaments of the liver become very prominent
Urogenital system: mesonephros is at the height of its differentiation. Metanephric collecting tubules begin branching. Earliest metanephric secretory tubules differentiating. Bladder-urethra separates from the rectum. The urethral membrane is beginning to rupture
Vascular system: cardinal veins are transforming. The inferior vena cava is visible. The atrium, ventricle, and bulbus are partitioned. Cardiac valves are present. Stem of the pulmonary vein is absorbed into the left atrium. The anlage of the spleen is prominent
Skeletal system: the chondrocranium is seen. Chondrification is now more general
Muscular system: muscles are differentiating rapidly throughout body and are assuming their final shapes and relationships
Integumentary system: there is mammary thickening
Nervous system: cerebral hemispheres are becoming large. Corpus striatum and thalamus are prominent. Infundibulum and Rathke's pouch are in contact. Choroid plexuses are appearing. Suprarenal medulla begins to invade the cortex
Sense organs: choroid fissure closes, enclosing the central artery. Nerve fibers invade the optic stalk. The lens loses its cavity by elongating lens fibers. Eyelids are forming. The fibrous and vascular coats of the eye are beginning to form. The olfactory sacs open into the mouth

Age (weeks): 8.0

SIZE (CR in mm): 23.0

Body form: nose is flat. Eyes are far apart. Digits are well formed. Growth of gut makes body evenly rotund. Head is elevating. The fetal state is now reached
Mouth: tongue muscles are well differentiated. The earliest taste buds are indicated. Rathke's pouch detaches from the mouth. The sublingual glands are now appearing
Pharynx and derivatives: the auditory tube and tympanic cavity are evident. The sites of the tonsils and their fossae are evident. The thymic gland halves unite and become solid. The thyroid gland follicles are forming
Digestive tube and glands: small intestine is coiling within the cord. The intestinal villi are developing. The liver is very large in relative size
Respiratory system: the lung is becoming glandlike by branching of the bronchioles
Coelom and mesenteries: pleuroperitoneal communications close. Pericardium is a very large sac. The diaphragm is completed, including its musculature. The diaphragm also completes its descent
Urogenital system: the testis and ovary are now distinguishable as such. Mullerian ducts are nearing the urogenital sinus and are about ready to unite with the uterovaginal primordium. The genital ligaments are indicated
Vascular system: the main blood vessels are assuming their final plan. The primitive lymph sacs are present. The sinus venosus is absorbed into the right atrium. The atrioventricular bundle is present
Skeletal system: the first indications of ossification are evident
Muscular system: definitive muscles of the trunk, limbs, and head are well represented, and .the fetus is now capable of some movement
Integumentary system: mammary primordia are seen as globular thickenings
Nervous system: the cerebral cortex begins to acquire its typical cells. The olfactory lobes are visible. The dura and pia-arachnoid are distinct. Chromaffin bodies are seen
Sense organs: the eyes are converging rapidly. The external, middle, and internal ears are assuming their final form. The taste buds are appearing. The external nares are plugged.

V Age (weeks): 10.0

SIZE (CR in mm): 40.0

Body form: head is erect. Limbs are well modeled. Nailfolds are indicated. The umbilical hernia is reduced
Mouth: fungiform and vallate papillae are differentiating. The lips are separate from the jaws. The enamel organs and dental papillae are forming. The palate folds are fusing
Pharynx and derivatives: the thymic epithelium is transforming into reticulum and thymic corpuscles. The ultimobranchial bodies disappear as such
Digestive tube and glands: the intestines withdraw from the umbilical cord and assume their characteristic position. The anal canal is formed. The pancreatic alveoli are present
Respiratory system: the nasal passages are partitioned by fusion of the septum and palate. The nose is cartilaginous. The laryngeal cavity is reopened and the vocal folds appear
Coelom and mesenteries: the processus (saccus) vaginales are forming. The intestine and its mesentery withdraw from the umbilical cord
Urogenital system: the kidneys are able to secrete. The bladder expands as a sac. The genital ducts of the opposite sex are degenerating. The bulbourethral and vestibular glands are appearing. The vagina sacs are forming
Vascular system: the thoracic duct and peripheral lymphatics are developed. Early lymph glands are appearing. Enucleated red blood cells predominate in the blood
Skeletal system: ossification centers are more common. The chondrocranium is at the height of its development
Muscular system: the perineal muscles are developing slowly
Integumentary system: intermediate cells are added to the epidermis. The periderm cells are prominent. The nail fields are indicated. The earliest hair follicles begin developing on the face
Nervous system: the spinal cord attains its definitive internal structure
Sense organs: the iris and ciliary bodies are organizing. The eyelids are fused. The lacrimal glands are budding. The spiral organ of Corti begins to differentiate

Age (weeks): 12.0

SIZE (CR in mm): 56.0

Body form: the head is still a dominant feature. The nose acquires its bridge. Sex is readily determined by external inspection
Mouth: the filiform and foliate papillae are elevating. Tooth primordia form prominent cups. The cheeks are seen. Fusion of the palate is complete
Pharynx and derivatives: the tonsillar crypts begin to invaginate. The thymus forms its medulla and is becoming more lymphoid. The thyroid attains its typical form
Digestive tube and glands: muscle layers of the gut are present. Pancreatic islets are appearing. Bile is being secreted
Respiratory system: the nasal conchae are prominent. Glands of the nasal cavity are forming. The lungs are acquiring their definitive form
Coelom and mesenteries: the omentum has become an expansive apron which is partly fused to the dorsal body wall. The mesenteries are free and exhibit their usual relationships. Extension of the coelom into the umbilical cord is obliterated
Urogenital system: the uterine horns are absorbed. The external genitalia attain distinctive features. The mesonephric and rete tubules complete the male duct. The prostate and seminal vesicles begin to appear. The hollow viscera are beginning to form muscular walls
Vascular system: blood formation is beginning in the bone marrow. The blood vessels acquire accessory coats
Skeletal system: the notochord is degenerating very rapidly. Ossification is spreading rapidly. A number of bones are well defined
Muscular system: smooth muscle layers are becoming evident in the hollow viscera
Integumentary system: the epidermis is now 3-layered. The corium and subcutaneous tissue are now distinct
Nervous system: the brain attains its general structural features. The spinal cord demonstrates cervical and lumbar enlargements. The cauda equina and filum terminale make their appearance. Neuroglial types are beginning to differentiate
Sense organs: the characteristic organization of the eye is attained. The retina is now becoming layered. The nasal septum and plate fusions are completed

Age (weeks): 16.0

SIZE (CR in mm): 112.0

Body form: face takes on a "human" appearance. Hair of the head is appearing. Muscles become spontaneously active. Body is beginning to outgrow head
Mouth: hard and soft palates are differentiating. The hypophysis is acquiring its definitive structure
Pharynx and derivatives: lymphocytes are beginning to accumulate in the tonsils. The pharyngeal tonsil (adenoids) is beginning to develop
Digestive tube and glands: gastric and intestinal glands are developing. The duodenum and colon become affixed to the posterior abdominal wall. Meconium is collecting
Respiratory system: the accessory nasal sinuses are developing. The tracheal glands appear. Mesoderm is still abundant between the pulmonary alveoli. Elastic fibers make their appearance in the lungs
Coelom and mesenteries: greater omentum is fusing with the transverse mesocolon and colon. The mesoduodenum, ascending mesocolon, and descending mesocolon are attaching to the posterior abdominal wall
Urogenital system: the kidneys attain their characteristic shape. The testis is in position for its descent into the scrotum. The uterus and vagina are recognizable as entities. The mesonephros is involuted
Vascular system: blood formation is now active in the spleen. The heart musculature is much condensed
Skeletal system: most bones are clearly indicated throughout the body. Joint cavities appear
Muscular system: cardiac muscle which appeared in earlier weeks is now more condensed. Muscular movements in utero can be detected
Integumentary system: the epidermis begins adding additional layers. The body hair begins to develop. The sweat glands appear. The first sebaceous glands begin to differentiate
Nervous system: the hemispheres now conceal much of the brain. The cerebral lobes are delimited. The corpora quadrigemina appear. The cerebellum attains some prominence
Sense organs: the eye, ear, and nose are nearing their typical appearance. The general sense organs are differentiating

X Age (weeks): 20.0-40.0

SIZE (CR in mm): 160.0-350.0

Body form: lanugo hair appears in week 20. Vernix caseosa collects in week 20. Body becomes better proportioned but lean in week 24. Fetus is wrinkled, lean, and red, and eyelids reopen in week 28. Testes invade the scrotum in week 32. Body rounds out, fat collects, and wrinkling smoothens out in weeks 32 to 40
Mouth: enamel and dentine deposited in week 20. Lingual tonsil forming in week 20. Permanent teeth primordia indicated in weeks 24 to 32. Milk teeth are unerupted at birth
Pharynx and derivatives: the tonsils are structurally typical in week 20
Digestive tube and glands: lymph nodules and muscularis mucosae of the gut are present in week 20. The ascending colon becomes recognizable in week 24. The appendix lags behind the cecum in growth at week 24. The deep esophageal glands are indicated in week 28. The plicae circulares are represented in week 32
Respiratory system: the nose begins ossifying in week 20. The nostrils reopen in week 24. The cuboidal epithelium of the lung alveoli is disappearing in week 24. Pulmonary branching is only two-thirds completed by week 40. The frontal and sphenoidal sinuses are still quite incomplete by week 40
Coelom and mesenteries: the mesenterial attachments are completed by week 20. The vaginal sacs are passing into the scrotal sacs between weeks 28 and 36
Urogenital system: the female urogenital sinus is becoming a shallow vestibule by week 20. The vagina regains its lumen by week 20. The uterine glands appear in week 28. The scrotum is solid until sacs and testes descend in weeks 28 to 36. The tubules of the kidney cease forming at birth
Vascular system: blood formation is increasing in the bone marrow but is decreasing in the liver between weeks 20 and 40. The spleen acquires its typical structure by week 28. A number of fetal blood vessels are discontinued by week 40
Skeletal system: the carpal, tarsal, and sternal bones ossify late, some after birth. Most epiphyseal centers appear after birth, many during adolescence
Muscular system: the perineal muscles finish their development by week 24
Integumentary system: the vernix caseosa is seen in week 20. The epidermis cornifies by week 20. The nail plates begin in week 20. Hairs emerge by week 24. The mammary primordia bud in week 20, and the buds hollow out and branch by week 32. The nail reaches the fingertip by week 36. Lanugo hair is prominent in week 28 and is shed in week 40
Nervous system: the commissures are completed by week 20. Myelinization of the cord begins in week 20. The cerebral cortex is typically layered by week 24. The cerebral fissures and convolutions appear rapidly in week 28. Myelinization of the brain begins in week 40
Sense organs: the nose and ear ossify in week 20. The vascular tunic of the lens is at its greatest by week 28. The retinal layers are completed and light perception is possible by week 28. Taste sense is present in week 32. The eyelids reopen between 28 and 32. The mastoid cells are still unformed by week 40. At birth, the ear is still deaf to sounds