Timeline human development

Embryology - 1 Dec 2023        Expand to Translate
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Introduction

This page is organised to show a week by week human timeline of development features and approximate timing of key events with more detailed information about specific events in different systems. For a less detailed timeline see week by week.

From a single cell	to a newborn infant	in 9 months.

Embryonic Development

• "Weeks" refer to embryonic development from fertilization.
  • Clinical weeks (shown in brackets) or Gestational Age GA) is from the first day of the Last Menstrual Period (LMP).
• "Stages" refer to the Carnegie stages of development.
• "Timing" refers to days from fertilization or post conception age (PC), not the clinical or gestational age (GA) calculated from LMP (add 2 weeks).
• Dates and staging are also "ideal", and there is significant biological variability in the general timing of events.
• Week 1 to Week 8 (GA 10)are considered the embryonic period of development.
• Week 9 to week 37 (GA 11-39) or birth are considered the fetal period of development.
• First month (4 weeks) after birth is the neonatal period of development.

Each developmental feature is linked to online content with more detailed information and resources such as images and movies. The superscript numbers are the original source references. There are similar "timelines" for other species shown below.

First Trimester

Week:	1	2	3	4	5	6	7	8
Carnegie stage:	1 2 3 4	5 6	7 8 9	10 11 12 13	14 15	16 17	18 19	20 21 22 23

First Trimester Timeline 

Links: human timeline | first trimester timeline | second trimester timeline | third trimester timeline
Gestational Day GA	Stage	Event
1	Menstrual Phase	menstrual cycle changes: uterus endometrium (loss), ovary (follicle development)
2
3
4
5	Proliferative Phase	menstrual cycle changes: uterus endometrium (proliferation), ovary (Follicle Development)
6
7
8	Proliferative Phase
9		menstrual cycle - Mid proliferative
10
11
12
13		menstrual cycle - Late Proliferative
14	ovulation Capacitation
Fertilization Day	Stage	Event
1	Secretory Phase Stage 1	fertilization, zygote, Secretory Phase
2	Stage 2	morula, Blastula
3		blastocyst
4	Stage 3	blastocyst Hatching (zona pellucida lost)
5		Late Secretory, blastocyst (free floating)
6	Stage 4	Adplantation
7	Stage 5
8		implantation
9
10
11
12
13	Stage 6	Chorionic Cavity
14
Day	Stage	Event
15
16	Stage 7	gastrulation, ectoderm, mesoderm, endoderm
17
18	Stage 8	neural neurogenesis, neural groove and folds are first seen
19
20	Stage 9	Musculoskeletal somitogenesis, first somites form and continue to be added in sequence caudally (1 - 3 somite pairs). neural the three main divisions of the brain, which are not cerebral vesicles, can be distinguished while the neural groove is still completely open Neural Crest mesencephalic neural crest is visible
21		heart cardiogenesis, week 3 begins as paired heart tubes.
Day	Stage	Event
22	Stage 10	Neural Crest differentiation at spinal cord level from day 22 until day 26 neural folds begin to fuse near the junction between brain and spinal cord, when Neural Crest cells are arising mainly from the neural ectoderm Neural Crest trigeminal, facial, and postotic ganglia components visible Neural Crest migration of vagal level neural crest cells begins (7-10 somite stage) neural rostral neural tube forms 3 primary brain vesicles (week 4) respiratory Week 4 - laryngotracheal groove forms on floor foregut.
23		heart begins to beat in Humans by day 22-23, first functioning embryonic organ formed.
24	Stage 11	thyroid - thyroid median endodermal thickening in the floor of pharynx neural rostral (or cephalic) neuropore closes within a few hours; closure is bidirectional, it takes place from the dorsal and terminal lips and may occur in two areas simultaneously. The two lips, however, behave differently. ventricular Optic ventricle appears and the neural groove/tube space is initially filled with amniotic fluid.
25	Stage 12	pituitary Week 4 hypophysial pouch, Rathke's pouch, diverticulum from roof liver septum transversum forming liver stroma and hepatic diverticulum forming hepatic trabeculae neural caudal neuropore takes a day to close (closure is approximately at future somitic pair 31/sacral vertebra 2) neural secondary neurulation begins ventricular onset of the ventricular system and separates the ependymal from the amniotic fluid. neural crest cardiac crest, neural crest from rhombomeres 6 and 7 that migrates to pharyngeal arch 3 and from there the truncus arteriosus neural crest vagal neural crest enter the foregut (20-25 somite stage)
26
27
28	Stage 13	neural the neural tube is normally completely closed, ventricular system now separated from amniotic fluid. Neural crest at spinal level is segregating, and spinal ganglia are in series with the somites. Spinal cord ventral roots beginning to develop. telencephalon cavity appears Neural - Vascular Development - hindbrain is supplied by two parallel neural arteries (or channels) that obtain their blood supply from carotid-vertebrobasilar anastomoses given by the pharyngeal arch arteries; trigeminal artery, the otic artery, hypoglossal artery, and the proatlantal artery. liver epithelial cord proliferation enmeshing stromal capillaries smell Crest comes from the nasal plates integumentary 4 weeks - simple ectoderm epithelium over mesenchyme integumentary 1-3 months ectoderm- germinative (basal) cell repeated division of generates stratified epithelium; mesoderm- differentiates into connective tissue and blood vessels vision Optic vesicle lies close to the surface ectoderm. The surface ectoderm overlying the optic vesicle, in response to this contact, has thickened to form the lens placode. Diaphragm - pleuroperitoneal fold (PPF) first discernible in human embryos (CRL 6mm).
29		pituitary Week 5 elongation, contacts infundibulum, diverticulum of diencephalon heart Week 5 septation starts, atrial and ventricular respiratory Week 5 left and right lung buds push into the pericardioperitoneal canals (primordia of pleural cavity) Respiratory Week 5 to 17 lung histology - pseudoglandular hearing Week 5 cochlear part of otic vesicle elongates (humans 2.5 turns)
30
31
32	Stage 14	Placodes sensory placodes, lens pit, otocyst, nasal placode, primary/secondary vesicles, fourth ventricle of brain somite continued segmentation of paraxial mesoderm (somite pairs), heart prominence head 1st, 2nd and 3rd pharyngeal arch, forebrain, site of lens placode, site of otic placode, stomodeum Body - heart, liver, umbilical cord, mesonephric ridge visible externally as bulges. limb upper and lower limb buds growing. Abdominal Wall mesoderm of the primary body wall coalesced in the ventral midline to create the abdominal cavity. neural first appearance of the future cerebral hemispheres. Cerebellar plate differentiated to an intermediate layer, and future rhombic lip identifiable Neural - Vascular Development - basilar artery forms from the consolidation of the neural arteries. ventricular Subarachnoid space initially as irregular spaces on the ventral surface of the spinal cord. liver hepatic gland and its vascular channels enlarge, hematopoietic function appears vision lens placode is indented by the lens pit.
33	Stage 15	neural cranial nerves (except olfactory and optic) are identifiable in more advanced embryos Neural - Vascular Development - vertebral arteries form from transverse anastomoses between cervical intersegmental arteries, beginning with the proatlantal artery and proceeding downward to the 6th intersegmental artery, vision lens pit is closed. The lens vesicle and optic cup lie close to the surface ectoderm and appear to press against the surface.
34
35		vision 35 to 37 days retinal pigment present
36		pituitary Week 6 - connecting stalk between pouch and oral cavity degenerates parathyroid Week 6 - diverticulum elongate, hollow then solid, dorsal cell proliferation thymus Week 6 - diverticulum elongate, hollow then solid, ventral cell proliferation adrenal Week 6 - fetal cortex forms from mesothelium adjacent to dorsal mesentery, medulla neural crest cells from adjacent sympathetic ganglia respiratory Week 6 - descent of heart and lungs into thorax. Pleuroperitoneal foramen closes tongue Week 6 - gustatory papilla, caudal midline near the foramen caecum (week 6 to 7 - nerve fibers approach the lingual epithelium)
37	Stage 16	Neural first parasympathetic ganglia, submandibular and ciliary, are identifiable Neural - Vascular Development - development of the middle cerebral artery is first identified as small buds originating proximal to the anterior cerebral artery on the anterior division of the primitive internal carotid artery. limb upper limb bud nerves median nerve, radial nerve and ulnar nerve entered into hand plate, myoblasts spindle shaped and oriented parallel to limb bud axis. Abdominal Wall muscle cell migration about 25% of the hemicircumference of the abdominal cavity, the lateral plate mesoderm has become more condensed and thicker in the area around the myoblasts. heart outflow tract elliptical configuration with four cushions, the two larger fusing at this stage. Semilunar valve leaflets form at the downstream end of the cushions head lip and palate components of the upper lip, medial nasal prominence and maxillary process present, median palatine process appears. Eyelid prior to the development of the eyelids, one small sulcus or groove forms above the eye (eyelid groove) and another below it.
38
39
40
41	Stage 17	neural telencephalon areas of the future archicortex, paleocortex, and neocortex, visible. Beginning of future choroid plexus ventricular primordium of the epidural space appears first on the ventral part of the vertebral canal and develops rostro-caudally smell olfactory nerve fibres enter the brain Eyelid sulcus (groove) above and below eye deepen and eyelid folds develop (below first and then above) Diaphragm - pleuroperitoneal fold (PPF) no longer separated from the diaphragm (CRL 14mm) Abdominal Wall muscle cells now migrated approximately 50% of the distance to the ventral midline, inner and outer layers were not discernible yet.
42		heart separation of common cardiac outflow (aortic arch and pulmonary aorta)
Day	Stage	Event
43		pancreas Week 7 to 20 pancreatic hormones secretion increases, small amount maternal insulin respiratory Week 7 - enlargement of liver stops descent of heart and lungs
44	Stage 18	limb bone forms by endochondrial ossification and throughout embryo replacement of cartilage with bone (week 5-12). neural smell vomeronasal fibres and nervus terminalis liverobturation due to epithelial proliferation, bile ducts became reorganized, continuity between liver cells and gut ventricular duramater appears and spaces surround the circumference of the spinal cord, which coalesce and contain many blood vessels. Female uterus opening of the paramesonephric (Müllerian) duct to the coelomic cavity formed as an invagination of the coelomic epithelium Abdominal Wall separation of the myoblasts into distinct inner and outer layers, with unidirectional orientation. Abdominal wall thicker in the region where secondary structures were forming compared with the primary body wall region, dorsally outermost layer of connective tissue approximately half of this thickness.
45		liver (stage 18 to 23) biliary ductules developed in periportal connective tissue produces ductal plates that receive biliary capillaries
46
47
48	Stage 19	vision - (stage 19 -22) eyelid folds develop into the eyelids and cover more of the eye as the palpebral fissure takes shape. The upper and the lower eyelids meet at the outer canthus in Stage 19. cardiovascular arterial system Chapter 18 fig. 447). aortic arches stages 11–19 (figs. 29–40). heart fusion of aortic and mitral endocardial cushion material Respiratory - first generation of subsegmental bronchi now complete, see bronchial tree reconstruction (plates 3 and 4). gastrointestinal tract - anal membrane defined. renal - Cloacal membrane ruptures from urinary pressure at stage 18 or stage 19, genital testis - Rete testis develops from the seminiferous cords at stages 19–23, and tunica albuginea forms.. ovary - Rete ovarii cords are developing. uterus Müllerian duct grows independently from the invagination of the coelomic epithelium during stages 19-23 musculoskeletal Sternum right and left sternal bars are present. (figs. 7-17 and 7-22) Abdominal Wall segregation of the myoblasts into four distinct muscle groups with unidirectional orientation of myoblasts. Myoblast migrated over half of the distance to the ventral midline, abdominal wall thickest where the muscles migrated and the outermost layer of connective tissue comprises approximately half of the total thickness of the abdominal wall. Rectus muscle completely separated after migrating over half the distance to the midline. neural rhombencephalon migration for olivary and arcuate nuclei begins. choroid plexus of the fourth ventricle present. stria medullaris thalami reaches the habenular nuclei. habenular commissure begins to develop. accessory olivary nucleus appears Neural - Vascular Development - middle cerebral artery becomes more prominent, the plexi fuse into a single artery and further branches pierce the cerebral hemisphere.
49
50	Stage 20	Head scalp vascular plexus visible limb upper limbs begin to rotate ventrally neural amygdaloid body has at least four individual nuclei oculomotor nerve shows a dorsolateral and a ventromedial portion rhombic lip (rhombencephalon) formation of the cerebellum (intermediate layer) and of the cochlear nuclei cerebellum cell layer (future Purkinje cells) develops choroid plexuses of the fourth and lateral ventricles Eyelid the inner canthus is established.
51		gastrointestinal tract anal membrane perforates
52	Stage 21	neural cortical plate appears in the area of future insula Neural - Vascular Development - formation of the anterior communicating artery. limb upper and lower limbs rotate Intraembryonic Coelom pericardioperitoneal canals close Abdominal Wall Myoblasts have reached the ventral midline and myotubes were present and oriented uniformly within all muscle groups. The rectus abdominis formed distinct bundles of muscle. Connective tissue layers comprised the majority of the thickness of the abdominal wall, outermost layer of connective tissue accounted for the majority of this thickness.
53
54	Stage 22	neural neocortical fibres project to epithalamus, to dorsal thalamus, and to mesencephalon limb fingers and toes lengthen smell Stage 22 to early fetal period - migratory streams of neurons from the subventricular zone of the olfactory bulb towards the future claustrum Uterus Vagina fused duct (uterovaginal canal) bifurcated at the caudal portion at Carnegie stages 22 and 23
55		Genital 8 Weeks Testis - mesenchyme, interstitial cells (of Leydig) secrete testosterone, androstenedione Genital 8 to 12 Weeks - hCG stimulates testosterone production Tongue Week 8 - nerves penetrate epitheilai basal lamina and synapse with undifferentiated, elongated, epithelial cells (taste bud progenitor cell)
56	Stage 23	Stage 23 defines the end of the embryonic (organogenesis) period Mesoderm heart prominence, ossification continues Head nose, eye, external acoustic meatus, eyelids, external ears, rounded head Body - straightening of trunk, umbilical cord, intestines herniated at umbilicus limb upper limbs longer and bent at elbow, hands and feet turned inward, foot with separated digits, wrist, hand with separated digits Extraembryonic Coelom chorionic cavity is now lost by fusion with the expanding amniotic cavity neural rhombencephalon, pyramidal decussation present, nuclei and tracts similar to those present in the newborn cerebellum present as only a plate connected to midbrain and hindbrain through fibre bundles Axial Skeleton vertebral column 33 or 34 cartilaginous vertebrae (20-33 mm in total length), vertebral pedicles, articular and transverse processes identifiable (no spinous processes) Abdominal Wall Rectus muscle forms 2 or 3 distinct layers with myotube orientation uniform in all muscles. The external oblique and internal oblique started to expand in thickness, transversus a thin layer of muscle.
	Week 8	Stomach Week 8 - Gastrin containing cells in stomach antrum. Somatostatin cells in both the antrum and the fundus. Genital - Female Development paired paramesonephric (Müllerian) ducts contact each other and are fused into a single tube that separates again and returns to the mesonephric (Wolffian) ducts. The paramesonephric ducts have not yet reached the urogenital sinus.
57-63	Week 9	Beginning of Fetal Development Historic Embryology In 1949 the embryologist George Streeter used the replacement of cartilage within the humerus by bone marrow as an arbitrary definition of the embryo to fetus transition. "If the onset can be recognized in a given specimen, that specimen is straightway classed as a fetus." CRL 43 mm, femur length 6 mm 9 weeks CRL 50 mm - genital genitalia in both sexes look identical uterus - paramesonephric ducts come into apposition with the urorectal septum and begin to fuse
Day	Stage	Event
64		Gastrointestinal Tract Week 10 intestines in abdomen Pituitary growth hormone and ACTH detectable Pancreas Week 10 glucagon (alpha) differentiate first, somatostatin (delta), insulin (beta) cells differentiate, insulin secretion begins Tongue Week 10 shallow grooves above the taste bud primordium Stomach Week 10 - Glucagon containing cells in stomach fundus. Nail Development fingernails appear outer ear Week 10 - Meatal plug extends in a disc-like fashion, the meatus is boot-shaped with a narrow neck and the sole of the meatal plug spreading widely to form the future tympanic membrane medially. Proximal portion of the neck starts to be resorbed. inner ear Week 10 - neural-crest-derived melanocytes migrate into the cochlea. They penetrate the basement membrane of the lateral wall epithelium and develop into the intermediate cells of the stria vascularis.
65
66
67
68
69
70		Week 10 - CRL 55 mm, femur length 9 mm, biparietal diameter 17 mm
Day	Stage	Event neural - Cerebrum appearance of the first sulcus (week 11-15, GA 13-17 weeks)
71		Thyroid colloid appearance in thyroid follicles, iodine and thyroid hormone (TH) synthesis Stomach Week 11 - Serotonin containing cells in both the antrum and the fundus.
72
73
74
75
76
77		Week 11 - CRL 68 mm, femur length 12 mm, biparietal diameter 20 mm
	Systems   Systems: bone timeline | eye neural crest timeline | heart abnormality timeline | hearing EAM timeline | muscle timeline | ovary timeline | placental villi timeline | shoulder timeline | smell timeline | spleen timeline | ventricular timeline

Second Trimester

(GA Clinical Week 14) Second Trimester

Second Trimester Timeline 

Links: human timeline | first trimester timeline | second trimester timeline | third trimester timeline
Week	Stage	Event
12	Clinical second trimester	Week 12 - CRL 85 mm, femur length 15 mm, biparietal diameter 25 mm Hearing Week 12-16 - Capsule adjacent to membranous labrynth undegoes vacuolization to form a cavity (perilymphatic space) around membranous labrynth and fills with perilymph Genital male and female external genital differences observable Respiratory Month 3-6 - lungs appear glandular, end month 6 alveolar cells type 2 appear and begin to secrete surfactant Tongue Week 12 - first differentiated epithelial cells (Type II and III) Genital female genital canal (80 days) formed with absorption of the median septum
13		tongue Week 12 to 13 - maximum synapses between cells and afferent nerve fibers hearing outer ear Week 13 - Meatal plug disc-like, innermost surface in contact with the primordial malleus, contributes to the formation of the tympanic membrane.
14		tongue Week 14 to 15 - taste pores develop, mucous ovary 100 days - primary follicles present nail toenails appear Head Development facial skeleton remodelling begins Hearing - Inner Ear Development Week 14 GA 16 - neural-crest-derived melanocytes, now intermediate cells of the stria vascularis, tightly integrate with Na+ /K+ -ATPase-positive marginal cells, which started to express KCNQ1 in their apical membrane.
15		Pancreas glucagon detectable in fetal plasma. spleen Week 15 -alpha-smooth muscle actin (alpha-SMA)-positive reticulum cells scattered around the arterioles.
16	14 cm	Hearing Week 16-24 - Centres of ossification appear in remaining cartilage of otic capsule form petrous portion of temporal bone. Continues to ossify to form mastoid process of temporal bone. pituitary adenohypophysis fully differentiated respiratory Week 16 to 25 lung histology - canalicular Hearing - Outer Ear Development Week 16.5 - External auditory meatus is fully patent throughout its length, lumen is still narrow and curved. Hearing - Inner Ear Development Week 16 GA 18 - cells in the outer sulcus express KCNJ10 and gap junction proteins GJB2/CX26 and GJB6/CX30, but these are not expressed in the spiral ligament. gap junction cartoon neural - Cerebrum development of the periinsular sulci (week 16-17, GA 18-19 weeks) integumentary 4 months - basal cell- proliferation generates folds in basement membrane; neural crest cells- (melanocytes) migrate into epithelium; embryonic connective tissue- differentiates into dermis, a loose ct layer over a dense ct layer. Beneath the dense ct layer is another loose ct layer that will form the subcutaneous layer. Ectoderm contributes to nails, hair follictles and glands. Nails form as thickening of ectoderm epidermis at the tips of fingers and toes. These form germinative cells of nail field. Cords of these cells extend into mesoderm forming epithelial columns. These form hair follocles, sebaceous and sweat glands. primary follicles begin to form in the ovary and are characterized by an oocyte glandular urethra forms and skin folds present
17		Neural - Brain development histology week 17 Cerebellum Magnetic Resonance Imaging (MRI) can study the developing cerebellum from 17 to 18 weeks (GA 19 to 20 weeks). tooth Week 17 - First papilla of the permanent dentition appear (first molar) immediately behind the second milk molar, milk teeth are well advanced (Fetus 180 mm).
18		tongue Week 18 - substance P detected in dermal papillae, not in taste bud primordia integumentary vernix caseosa covers skin spleen Week 18 - alpha-SMA-positive reticulum cells increase in number and began to form a reticular framework. An accumulation of T and B lymphocytes occurred within the framework, and a primitive white pulp was observed around the arterioles. Hearing - Outer Ear Development week 18 - External auditory meatus is already fully expanded to its complete form. neural - Cerebrum central sulci and opercularization of the insula (week 18-20, GA 20-22 weeks)
19		neural week 19 neuronal migration ends and the radial glial cells that aided the migration now become transformed into astrocytes and astrocytic precursors.
20		pituitary week 20 to 24 growth hormone levels peak, then decline integumentary lanugo, skin hair integumentary 5 months - Hair growth initiated at base of cord, lateral outgrowths form associated sebaceous glands; Other cords elongate and coil to form sweat glands; Cords in mammary region branch as they elongate to form mammary glands. touch pacinian corpuscle begin to develop
21
22		Neural brain cortical sulcation - sylvian fissure, interhemispheric fissure, callosal sulcus, parietooccipital fissure, and hippocampic fissures present spleen - Week 22 - antigenic diversity of the reticular framework was observed, and T and B lymphocytes were segregated in the framework. T lymphocytes were sorted into the alpha-smooth muscle actin-positive reticular framework, and the periarteriolar lymphoid sheath (PALS) was formed around the arteriole. B lymphocytes aggregated in eccentric portions to the PALS and formed the lymph follicle (LF). The reticular framework of the LF was alpha-SMA-negative. neural - Cerebrum covering of the posterior insula (week 22-24, GA 24-26 weeks)
23
24		respiratory Week 24 to 40 lung histology - terminal sac spleen Week 24 - marginal zone appeared in the alpha-smooth muscle actin-positive reticular framework around the white pulp. tooth Week 24 - Permanent incisors and canines appear. Earliest potential survival expected if born ovary follicles can consist of growing oocytes surrounded by several layers of granulosa cells
25		respiratory end month 6 alveolar cells type 2 appear and begin to secrete surfactant neural - Cerebrum closure of the laeteral sulcus (Sylvian fissure or lateral fissure) (week 25-26, GA 27-28 weeks)
26		touch pacinian corpuscle well developed
	Systems   Systems: bone timeline | eye neural crest timeline | heart abnormality timeline | hearing EAM timeline | muscle timeline | ovary timeline | placental villi timeline | shoulder timeline | smell timeline | spleen timeline | ventricular timeline

Third Trimester

Third Trimester Timeline

Links: human timeline | first trimester timeline | second trimester timeline | third trimester timeline
Week	Stage	Event
	Clinical third trimester	hearing 3rd Trimester - vibration acoustically of maternal abdominal wall induces startle respone in fetus.
27
28		respiratory Month 7 - respiratory bronchioles proliferate and end in alveolar ducts and sacs
29		tooth Week 29 - Permanent premolars (correspond to the milk molars) appear.
30		Genital male gonad (testes) descending
31
32		nail fingernails reach digit tip
33		neural brain cortical sulcation - primary sulci present
34		neural brain cortical sulcation - insular, cingular, and occipital secondary sulci present
35
36		Nail Development toenails reach digit tip Lens Development - lens growth and interocular distance plateaus after 36 weeks of gestation
37
38	Birth	Clinical Week 40 Heart pressure difference closes foramen ovale leaving a fossa ovalis thyroid TSH levels increase, thyroxine (T3) and T4 levels increase to 24 h, then 5-7 days postnatal decline to normal levels adrenal - zona glomerulosa, zona fasiculata present
	Systems   Systems: bone timeline | eye neural crest timeline | heart abnormality timeline | hearing EAM timeline | muscle timeline | ovary timeline | placental villi timeline | shoulder timeline | smell timeline | spleen timeline | ventricular timeline

Postnatal

Week	Stage	Event
+1		Normal Newborn Neural Exam Movies   Normal Newborn Neural Exam Movies The newborn neuromuscular system can be initially assessed by 6 quick tests (posture, square window, arm recoil, popliteal angle, scarf sign and heel to ear). The following short videos show clinical examination of these assessments and a number of neonatal reflexes. Later developmental assessment includes behaviour, reflexes (primitive and postural), muscular tone, and motor (gross, fine, co-ordination). Normal Newborn Neural Exam Movies Newborn Behaviour  ‎‎Behaviour Page | Play  ‎‎Cranial Nerves Page | Play Newborn Tone  ‎‎Resting Posture Page | Play ‎‎ Tone Upper Limb Page | Play ‎‎ Arm Traction Page | Play  ‎‎Arm Recoil Page | Play  ‎‎Scarf Sign Page | Play  ‎‎Hand Position Page | Play  ‎‎Lower Limb Page | Play  ‎‎Leg Traction Page | Play  ‎‎Leg Recoil Page | Play  ‎‎Popliteal Angle Page | Play  ‎‎Heel to Ear Page | Play  ‎‎Neck Tone Page | Play  ‎‎Head Lag Page | Play  ‎‎Head Control Page | Play Newborn Positions  ‎‎Prone Position Page | Play  ‎‎Ventral Suspend Page | Play  ‎‎Vertical Suspend Page | Play Newborn Reflexes  ‎‎Tendon Reflexes Page | Play  ‎‎Plantar Reflex Page | Play  ‎‎Reflex Suck,Root Page | Play  ‎‎Moro Reflex Page | Play  ‎‎Galant Reflex Page | Play  ‎‎Stepping Reflex Page | Play  ‎‎Grasp Reflex Page | Play Newborn Head ‎‎ Head Shape Page | Play ‎‎Head Circumference Page | Play Neural Exam Movies: normal behaviour | cranial nerves | Newborn Tone - resting posture | upper extremity‎ | arm traction | arm recoil | scarf sign | hand position | lower extremity | leg traction‎ | leg recoil‎‎ | popliteal angle‎ | heel to ear | neck tone | head lag‎ | head control | Newborn Positions - prone | ventral suspension | vertical suspension | Newborn Reflexes - deep tendon reflexes | plantar reflex‎ | suck, root | Moro | Galant | stepping‎ | grasp | Newborn Head - head shape and sutures‎ | head circumference‎ | Neonatal Diagnosis
+2
+3
+4		vision - eye globe growth plateaus after 42 weeks of gestation
+5
Month		testis | spermatozoa - about 2 months of age, primordial germ cells (gonocytes) are replaced by adult dark (Ad) and pale (Ap) spermatogonia forming the spermatogonial stem cell (SSC) population that at puberty will commence differentiation into spermatozoa.
Year
Year 1		neural Hearing (6 months to 5 years) thalamocortical afferents to the deeper cortical layers mature and are the first source of input to the auditory cortex Normal 12 Month Neural Exam Movies   Normal 12 Month Neural Exam Movies The following short videos show clinical examination of neural development assessments and a number of reflexes. 12 Months Neural Exam Movies: Shy | Social and Language | Cranial Nerves | Tone - Tone | Reflexes - deep tendon reflexes | plantar reflex‎ | Parachute | Pincer Grasp | Beads in the Cup | Play Ball | Transition in and out of Sitting | Creeping | Stoop and Recover | Motor/Gait - Stand, Walks with Support | Toddler’s Gait | Head Circumference | Neural Exam Movies
Year 2
Year 3		Adrenal - Year 3 zona reticularis present. Normal 30 Month Neural Exam Movies   Normal 30 Month Neural Exam Movies The following short videos show clinical examination of neural development assessments and a number of reflexes. Neural Exam Movies: Establishing Relationship | Follows Commands | Points to Pictures | Names Pictures | Response to Questions | Pointing to and Naming Body Parts | Motor/Coordination - Using Puppets | Using Measuring Tape | Block Tower | Drawing | Tone | Deep Tendon Reflex | Kicking and Throwing a Ball | Walking, Running
4
5		neural hearing - (5 to 12 years) commissural and association axons in the superficial cortical layers allows communication between subdivisions of the auditory cortex
7-13		puberty - Female
9-15		puberty - Male

Human Systems

Somite

Human Embryo Somite Number

Week	Days	Carnegie Stage	Somite Number (pairs)
Week 3	19 - 21	9    image	1 - 3
Week 4	22 - 23	10   image	4 - 12
Week 4	23 - 26	11   image	13 - 20
Week 4	26 - 30	12   image	21 - 29
Week 5	28 - 32	13   image	30
Week 5	31 - 35	14   image	30+

Limb

Carnegie stages for limb elements appearance
	Skeletal Element	Condensed Mesenchyme	Cartilage	Bone
	Humerus	16	16 — 17	21 — 22
	Radius	16	17	21 — 23
	Ulna	16	17 — 18	17 — 23
	Hand	17	17 — 21	In fetus1and after birth
	Femur	17	17—18	22 — 23
	Patella	20	21	After birth
	Tibia	17	17 — 23	22 — 23
	Fibula	17	17 — 18	In fetus
	Foot	17 — 18	18 — 23 or later	in fetus and after birth
1 Intramembranous ossification at the tips of the distal phalanges of the hand may be in Stage 23.
Reference - O'Rahilly R. Gray DI. and Gardner E. Chondrification in the hands and feet of staged human embryos. (1957) Carnegie Instn. Wash. Publ. 611, Contrib. Embryol., 36:

Neural

Early Neural Timeline
Carnegie Stage	Event
8	(about 18 postovulatory days) neural groove and folds are first seen
9	three main divisions of the brain, which are not cerebral vesicles, can be distinguished while the neural groove is still completely open.
10	(two days later) neural folds begin to fuse near the junction between brain and spinal cord, when neural crest cells are arising mainly from the neural ectoderm
11	(about 24 days) the rostral (or cephalic) neuropore closes within a few hours; closure is bidirectional, it takes place from the dorsal and terminal lips and may occur in several areas simultaneously. The two lips, however, behave differently.
12	(about 26 days) The caudal neuropore takes a day to close. The level of final closure is approximately at future somitic pair 31 (corresponds to the level of sacral vertebra 2). Secondary neurulation begins, is the differentiation of the caudal part of the neural tube from the caudal eminence (or end-bud) without the intermediate phase of a neural plate.
13	(4 weeks) the neural tube is normally completely closed.
Links: neural | Week 3 | Week 4

Neural timeline overview

Sensorimotor Cortex

Sensorimotor Cortex Timeline
Gestational Age GA weeks	Fertilization Age FA weeks	Event
18-19	16-17	appearance of the inferior part of the central cerebral sulcus
20-22	18-20	development of the pericentral lateral regions and the beginning of opercularization
24-26	22-24	development of parietal and temporal cortices and the covering of the postcentral insular region
27-28	25-26	maturation of the central cerebral regions
Table data.    Links: sensorimotor cortex | cerebral cortex | second trimester | Sensory System Development

Notochord

Human Notochord Timeline

Carnegie Stage	Days	Event
7	15 to 17	notochordal process directly rostral to the area of gastrulation.
8 to 10	17 to 23	group of undifferentiated cells that briefly persists. Within the notochordal process, a connection between the amniotic cavity and yolk-sac (neurenteric canal).
8 to 11	17 to 26	notochordal process incorporates entirely into the endoderm, forming the epithelial notochordal plate.
10 to 11	21 to 26	notochordal plate then acquires an “inverted U-shape”, relatively lengthens during these stages and remains intimately associated with the neural tube until stage 12 (26–30 days).
12	26 to 30	notochordal plate detaches completely from the endoderm to form the definitive notochord.
12 to 14	26 to 35	definitive notochord formation then allows the paired dorsal aortae to fuse in-between the notochord and the roof of the gut.
		Table data   Links: notochord | timeline | Week 3 | Week 4

Ventricular

Human Embryonic Ventricular Timeline

Week	Carnegie Stage	Event
Week 4	11	appearance of the optic ventricle. The neural groove/tube space is initially filled with amniotic fluid.
	12	closure of the caudal neuropore, onset of the ventricular system and separates the ependymal from the amniotic fluid.
	13	cavity of the telencephalon medium is visible.
Week 5	14	cerebral hemispheres and lateral ventricles begin, rhomboid fossa becomes apparent.
	15	medial and lateral ventricular eminences cause indentations in the lateral ventricle
Week 6	16	hypothalamic sulcus is evident.
	17 - 18	interventricular foramina are becoming relatively smaller, and cellular accumulations indicate the future choroid villi of the fourth and lateral ventricles.
Week 7	18	areae membranaceae rostralis and caudalis are visible in the roof of the fourth ventricle, and the paraphysis is appearing.
	19	choroid villi are visible in the fourth ventricle, and a mesencephalic evagination (blindsack) is visible
Week 8	20	choroid villi are visible in the lateral ventricle.
	21	olfactory ventricle is visible.
	21 - 23	lateral ventricle has become C-shaped (anterior and inferior horns visible). Recesses develop in the third ventricle (optic, infundibular, pineal).
Week: 1 2 3 4 5 6 7 8 Carnegie stage: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
Links: ventricular | neural | timeline | Category:Timeline     Table Data Reference

Neural Crest

Human Eye Neural Crest Timeline

Carnegie Stage	Event
9	an indication of mesencephalic neural crest
10	trigeminal, facial, and postotic components
11	crest-free zones are soon observable in rhombomere 1, 3, and 5
12	rhombomeres 6 and 7 neural crest migrate to pharyngeal arch 3 and then rostrad to the truncus arteriosus
13	nasal crest and the terminalis-vomeronasal complex are last of the cranial crest to appear
9 to 14	otic vesicle primordium descends

Week:	1	2	3	4	5	6	7	8
Carnegie stage:	1 2 3 4	5 6	7 8 9	10 11 12 13	14 15	16 17	18 19	20 21 22 23

Data from a study of 185 serially sectioned staged (Carnegie) human embryos. Links: vision | neural crest | timeline |     Category:Timeline

Hearing

Hearing Timeline

Developmental Time	Event
Week 9	Mesenchyme surrounding membranous labryinth (otic capsule) chondrifies.
Week 12-16	Capsule adjacent to membranous labryinth undegoes vacuolization to form a cavity (perilymphatic space) around membranous labrynth and fills with perilymph.
Week 18	ectodermal plug in external auditory meatus breaks down.
Week 16 - 24	Centres of endochondral ossification appear in remaining cartilage of otic capsule form petrous portion of temporal bone. Continues to ossify to form mastoid process of temporal bone.
Week 18 - 22	Organ of corti structural elements develop. (GA 20 - 24 weeks)
26 weeks	human brainstem auditory pathway is anatomically formed.
28 weeks	AABR can be recorded.
3rd Trimester	Vibration acoustically of maternal abdominal wall induces startle response in fetus.
Links: hearing | timeline

Stapedius Timeline

Carnegie Stage	CRL (mm)	Description
13	6	Presumptive stapedial area
14	7	Appearance of the stapedial anlage
16	9	Relationship between the stapedial artery and the stapedial anlage. Appearance of the interhyale
17	12	Delimitation of the parts of the stapedial anlage
18	16	Chondrogenesis phase. Start of involution of the stapedial artery
20	18.5	Delimitation of the ossicular anlages. Cartilaginous phase. Disappearance of the stapedial artery
22	26	Delimitation of the interhyale
23	28	Anlage of the stapedial muscle tendon
Data from Table 1   Links: middle ear

External Auditory Meatus Timeline

Period	Week	Description
Embryo	week 8	Funnel-shaped tube continues medially into mesenchymal tissue, forms a curved path.
Fetus (first trimester)	week 9	Ectodermal cells proliferate, fill the meatus lumen and form the "meatal plug".
Fetus (first trimester)	week 10	Meatal plug bottom extends in a disc-like fashion, so that in the horizontal plane the meatus is boot-shaped with a narrow neck and the sole of the meatal plug spreading widely to form the future tympanic membrane medially. At the same time, the plug in the proximal portion of the neck starts to be resorbed.
Fetus (second trimester)	week 13	Meatal plug disc-like, innermost surface in contact with the primordial malleus, contributes to formation of tympanic membrane.
Fetus (second trimester)	week 15	Meatal plug innermost portion splits, leaving a thin ectodermal cell layer of immature tympanic membrane. The neck of the boot forms the border between the primary and secondary meatus, and is the last part to split.
Fetus (second trimester)	week 16.5	The meatus is fully patent throughout entire length. Lumen is still narrow and curved. Epithelium cornification commences.
Fetus (second trimester)	week 18	The meatus is now fully expanded to its complete form.
Links: outer ear | hearing | timeline     Reference

Vision

Human Eye Development

Carnegie Stage	Event
10	optic primordia appear.
11	Right and left optic primordia meet at the optic chiasma forming a U-shaped rim.
12	optic neural crest reaches its maximum extent and the optic vesicle becomes covered by a complete sheath,
13	By the end of the fourth week the optic vesicle lies close to the surface ectoderm. Optic evagination differentiation allows identification of optic part of retina, future pigmented layer of retina, and optic stalk. The surface ectoderm overlying the optic vesicle, in response to this contact, has thickened to form the lense placode.
14	(about 32 days) Lens placode is indented by the lens pit, cup-shaped and still communicates with the surface by a narrowing pore.
15	(about 33 days) Lens pit is closed. The lens vesicle and optic cup lie close to the surface ectoderm and appear to press against the surface.
16	(37 days) Growth of the lens body results in a D-shaped lens cavity. Perilental blood vessels (tunica vasculosa lentis) are visible. Prior to the development of the eyelids, one small sulcus or groove forms above the eye (eyelid groove) and another below it.
17 - 19	Retinal pigment is visible and the retinal fissure is largely closed. Eyelids grooves deepen, eyelid folds develop, first below, and then above, the eye.
18	Mesenchyme invades the region between the lens epithelium and the surface ectoderm.
19 - 22	Eyelid folds develop into the eyelids and cover more of the eye as the palpebral fissure takes shape. The upper and the lower eyelids meet at the outer canthus in Stage 19.
20	Lens cavity is lost and a lens suture begins to form. The inner canthus is established.
23	retina comprises the pigmented layer, external limiting membrane, proliferative zone, external neuroblastic layer, transient fiber layer, internal neuroblastic layer, nerve fiber layer, and internal limiting membrane. Eyelids closure is complete (Note - shown as still open in the Kyoto embryo).
Data from a study of human embryonic carnegie stages and other sources. Week: 1 2 3 4 5 6 7 8 Carnegie stage: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Human Eye Neural Crest Timeline

Carnegie Stage	Event
9	an indication of mesencephalic neural crest
10	trigeminal, facial, and postotic components
11	crest-free zones are soon observable in rhombomere 1, 3, and 5
12	rhombomeres 6 and 7 neural crest migrate to pharyngeal arch 3 and then rostrad to the truncus arteriosus
13	nasal crest and the terminalis-vomeronasal complex are last of the cranial crest to appear
9 to 14	otic vesicle primordium descends

Week:	1	2	3	4	5	6	7	8
Carnegie stage:	1 2 3 4	5 6	7 8 9	10 11 12 13	14 15	16 17	18 19	20 21 22 23

Data from a study of 185 serially sectioned staged (Carnegie) human embryos. Links: vision | neural crest | timeline |     Category:Timeline

Human Embryonic Eyelid Timeline

Carnegie Stage	Event
10	optic primordia appear
13	By the end of the fourth week the optic vesicle lies close to the surface ectoderm. The surface ectoderm overlying the optic vesicle, in response to this contact, has thickened to form the lense placode.
14	(about 32 days) the lens placode is indented by the lens pit.
15	(about 33 days) the lens pit is closed. The lens vesicle and optic cup lie close to the surface ectoderm and appear to press against the surface.
16	(37 days) Prior to the development of the eyelids, one small sulcus or groove forms above the eye (eyelid groove) and another below it.
17 - 19	grooves deepen, eyelid folds develop, first below, and then above, the eye.
19 - 22	eyelid folds develop into the eyelids and cover more of the eye as the palpebral fissure takes shape. The upper and the lower eyelids meet at the outer canthus in Stage 19.
20	the inner canthus is established.
23	closure of the eyelids is complete (Note - shown as still open in some Kyoto embryo).
	Data - from Kyoto embryos with Carnegie staging.   Links: eyelid

Smell

Embryonic Smell Development

Week FA (GA)	Carnegie Stage	Event
week 4 (GA 6)	11	nasal epiblastic thickening appears
	12	nasal field is well outlined
week 5 (GA 7)	15	continuous cellulovascular strand seen between the nasal groove and the olfactory field
week 6 (GA 8)	16	vomeronasal groove appears.
	17	olfactory nerve is organized into two plexuses, lateral and medial, the latter mingled with the terminal-vomeronasal complex.
week 7 (GA 9)	18	olfactory bulb begins to appear
	19	individualization of the olfactory bulb and nuclei, distinction between olfactory structures and terminal and vomeronasal ones begins to be clear.
week 8 (GA 10)	21	structure of the olfactory bulb is evident.
	23	olfactory strands are well individualized, and olfactory and terminal-vomeronasal fibers are easily distinguishable.
Links: smell | sensory | timeline | Category:Timeline    Table Data Reference

Cardiovascular

Heart

Human Heart Timeline

Characteristic	Carnegie stage:	13	14	15	16	17	18	19	20	21	22	23
Septum primum
Foramen primum
Atrioventricular bundle
Atrioventricular cushions
Conotruncal ridges
Foramen secundum
Semilunar cusps
Conotruncal septum; atria
Closure primum foramen
Fusion atrioventricular cushions
Septum secundum and foramen ovale
Closure secondary interventricular foramen
Chordae tendineae
Colour Coding:	beginning to appear		present			Table data    Links: heart | Madrid Collection

Table 1. Comparison of Human and Murine Development
Human			Mouse
Carnegie Stage	Post-ovulatory Days	Size (mm) Greatest Length	Theiler Stage	Post-conception Days	Size (mm) Crown-rump
11	24	2.5 - 4.5	14	9 - 9.5	2.1
12	26	3 - 5	15	9.5 - 10.25	2.5
13	28	4 - 6	16	10.25 - 10.5	3.6
14	32	5 - 7	17	10.5	4.1
15	33	7 - 9	18	11	4.6
16	37	8 - 11	19	11.5	6 - 7
17	41	11 - 14	20	12	7
18	44	13 - 17	21	12.5 - 13	8.2
19	47.5	16 - 18	21	12.5 - 13	8.2
20	50.5	18 - 22	22	13.5 - 14	9.1
21	52	22 - 24	22	13.5 - 14	9.1
22	54	23 - 28	22	13.5 - 14	9.1
23	56.5	27 - 31	22	13.5 - 14	9.1
			26	17.5 - 18	17.8
Notes: Human Embryonic Development | Carnegie Stages | Mouse Development | Theiler Stage | Post-conception Days | Carnegie Stage Comparison Reference: Anderson RH. Teratogenecity in the setting of cardiac development and maldevelopment. (2016)

Cerebral Arterial

Cerebral Arterial Timeline

Carnegie Stage	CRL (mm)	Event
13	4 - 5	hindbrain (future posterior fossa) is supplied by two parallel neural arteries (or channels). These arteries obtain their blood supply from carotid-vertebrobasilar anastomoses given by the trigeminal artery (TA), the otic artery (OA), hypoglossal artery (HA), and the proatlantal artery (ProA)
14	5 - 8	basilar artery (BA) forms from the consolidation of the neural arteries.
15	7 - 12	vertebral arteries (VA) forms from transverse anastomoses between cervical intersegmental arteries, beginning with the ProA and proceeding downward to the 6th intersegmental artery,
16	11 - 12	(35 days) development of the middle cerebral artery (MCA) is first identified as small buds originating proximal to the anterior cerebral artery (ACA) on the anterior division of the primitive internal carotid artery (ICA).
19	16 - 18	middle cerebral artery (MCA) becomes more prominent, the plexi fuse into a single artery and further branches pierce the cerebral hemisphere.
20	18	stem of the ACA gives rise to the olfactory artery.
21	21-24	formation of the anterior communicating artery (ACOMM).
Early development - the posterior circulation relies almost entirely from blood supply coming from the anterior circulation through carotid-vertebrobasilar anastomoses.
Later development - as the posterior fossa structures and the occipital lobe grow, the posterior circulation becomes progressively independent from the anterior circulation with obliteration of the anterior-posterior anastomoses from caudal to rostral maintaining in the majority of adult only one connection between the distal basilar arteries with the carotid artery via the posterior communicating artery.
Data source)  Links: neural vascular

Placenta

Human Villi Timeline

Fertilization Age (weeks)	Gestational Age (weeks)	Vessel Lumen Diameter (range in microns, µm)	Features
3 to 4	5 and 6	10 - 15	a complex network of cords and vessels with redundant connections network comprises mainly cords, already connected together vessels and cords are connected to each other without any interruptions chorionic villus dominated by this network of vascular elements vessels and cords are located centrally as well as peripherally and as a consequence contact the overlying trophoblastic layer
5 to 6	7 and 8	10 - 26	villi dominated by capillary network of vessels and cords capillary network contains more vessels than cords chorionic villus tip - regular small branched off (mesenchymal) chorionic villi are present containing CD31 positive cells
7 to 8	9 and 10	60 - 75 two central vessels 26 - 34 capillary network	villi have two large centrally located vessels surrounded by and connected to a peripheral capillary network capillary network contains vessels with a lumen in tight contact with overlying trophoblastic layer villous projections also contain blind ending capillary sprouts
9 to 10	11 and 12	70 - 90 two central vessels 26 - 34 capillary network	immature intermediate villi characterized by two large vessels surrounded by a capillary network capillary network has few cords blind ending capillary sprouts off the capillary network
Vill development data based upon immunochemistry confocal laser scanning microscope (CLSM) study with clinical gestational age (GA) from last menstrual period (LMP) and has been corrected for post-conception (fertilization) age, approximately 14 days later. CD31 - (PECAM-1, Platelet Endothelial Cell Adhesion Molecule) is a cluster of differentiation molecule found on endothelial and other blood cells.

Liver

Human Embryonic Liver Development

Week	Carnegie Stage	Feature
Week 4	Carnegie stage 11	hepatic diverticulum development (ductal plate)
	Carnegie stage 12	cell differentiation septum transversum forming liver stroma hepatic diverticulum forming hepatic trabeculae
	Carnegie stage 13	epithelial cord proliferation enmeshing stromal capillaries
Week 5	Carnegie stage 14	hepatic gland and its vascular channels enlarge hematopoietic function appeared
Week 7	Carnegie stage 18	obturation due to epithelial proliferation bile ducts became reorganized (continuity between liver cells and gut)
Week 7 to 8	Carnegie stage 18 to Carnegie stage 23	biliary ductules developed in periportal connective tissue produces ductal plates that receive biliary capillaries
Human data, see also liver development in the rat embryonic period (Carnegie stages 15-23). (More? Detailed Timeline | Timeline human development)

Embryonic Liver Development Timeline  

Carnegie Stage	Age (days)	CRL (mm)	Biliary system	Vascular	Hepatic parenchyma
14	33	7	Bile duct - primordial duct links primitive intestine and liver parenchyma. Thick-walled tube (95 µm diameter) small lumen (22 µm diameter). Gall bladder - elongated tube further dilated, thick wall (125 µm diameter) and a narrow lumen (43 µm diameter).	Hepatic sinusoids - intra-hepatic vasculature present Three venous tributaries flow into the liver sinusoids - right and left placental vein and a single vitelline vein.	Cords of liver cells fragmented by vascular network of hepatic sinusoids. Between pericardial cavity (top) and mesonephros (bottom). Upper pole of the liver lies close to the septum transversum and early ventricles. Liver occupies the majority of abdominal cavity.
18	46	15	Bile duct (future common bile duct), and a common hepatic duct, in contact with liver parenchyma without penetration. Primordium of accessory bile tract is an elongated and fusiform gall bladder projecting forward and by a short cystic duct that opens into common bile duct. Bile duct empties into second part of duodenum on its posterior side.	Portal system visible - portal vein (100 µm diameter) arises from connection of upper mesenteric vein then at region of hepatic hilum (285 µm) divides into portal branches. Left umbilical vein empties into anterior extremity of the left portal branch. Ductus venosus (80 µm) connects the initial portion of left portal vein to the inferior vena cava. Hepatic venous system 3 branches - left hepatic vein (120 µm in diameter), middle hepatic vein (220 􏰁µm in diameter) and right hepatic vein (160 µm in diameter). Flows into the sub-cardinal vein.	Liver parenchyma has two anatomical lobes (right and left lobe), separated by anteroposterior plane formed by placental vein.
21	53	22.5	Bile duct morphology as earlier stage. Common bile duct empties at the level of the proximal duodenum.	Portal vein arises from joining of splenic vein and superior mesenteric vein. At the level of the hepatic hilum, portal vein divides into two branches, right portal branch (420 µm in diameter) and left portal branch (540 µm in diameter). Right portal branch gives rise to a thin branch to caudate lobe. Ventral branch gives rise to segmental portal veins (VIII and V). Dorsal branch gives rise to the segmental portal veins (VI and VII). Ductus venosus connects initial portion of left portal vein to inferior vena cava, just upstream from hepatic vein afferents. Hepatic venous system as for previous stage.	Hepatic parenchyma a large rounded mass.
23	58	27	Bile duct morphology as earlier stage.	Portal venous system complete. Ductus venosus (40 µm) connects initial portion of portal vein to middle hepatic vein. hepatic venous system has changed very little from the previous stage. Three hepatic veins empty into inferior vena cava.	Liver parenchyma roughly oval shape, 2 symmetrical hepatic lobes. The quadrate and caudate lobes are identifiable. Upper pole of the liver bounded above by diaphragm.
Data from a recent human study Links: liver | Carnegie stage 14 | 18 | 21 | 23 | simple embryonic timeline | Timeline human development

Pancreas

Human Pancreas Timeline

Carnegie Stage	Days	Event
10	25-27	distal endoderm foregut
12	29-31	pancreatic duodenal endoderm extra-hepatic billiard duct
13	30-33	pancreatic bud
19	47	"trunk" progenitor "tip" progenitor
23	8 weeks +	fetal beta cell ductal cell
fetal	14 weeks	acinar cell
Table data   Links: pancreas | exocrine pancreas | pancreas molecular timeline | timeline

Human Pancreas Timeline
Human Pancreas Timeline Carnegie Stage Days Event 10 25-27 distal endoderm foregut 12 29-31 pancreatic duodenal endoderm extra-hepatic billiard duct 13 30-33 pancreatic bud 19 47 "trunk" progenitor "tip" progenitor 23 8 weeks + fetal beta cell ductal cell fetal 14 weeks acinar cell   Table data   Links: pancreas | exocrine pancreas | pancreas molecular timeline | timeline

Human Pancreas Timeline

Carnegie Stage	Days	Molecular	Event
10	25-27	FOXA2, SOX17 (dorsal), SHH (ventral)	distal endoderm foregut
12	29-31	PDX1, FOXA2, SOX17, GATA4, SOX9 (weak) SOX9, FOXA2, PDX1 (weak)	pancreatic duodenal endoderm extra-hepatic billiard duct
13	30-33	PDX1, FOXA2, GATA4, NKX6.1, SOX9	pancreatic bud
19	47	PDX1, FOXA2, GATA4, NKX6.1, SOX9 PDX1, FOXA2, GATA4, NKX6.1, SOX9	"trunk" progenitor "tip" progenitor
23	8 weeks +	PDX1, FOXA2, GATA4, NKX6.1, NKX2.2, ISL1 SOX9, FOXA2, PDX1 (weak)	fetal beta cell ductal cell
fetal	14 weeks	GATA4	acini cell
Table data   Links: pancreas | exocrine pancreas | timeline

Human Pancreas Molecular Timeline

Human Pancreas Timeline Carnegie Stage Days Molecular Event 10 25-27 FOXA2, SOX17 (dorsal), SHH (ventral) distal endoderm foregut 12 29-31 PDX1, FOXA2, SOX17, GATA4, SOX9 (weak) SOX9, FOXA2, PDX1 (weak) pancreatic duodenal endoderm extra-hepatic billiard duct 13 30-33 PDX1, FOXA2, GATA4, NKX6.1, SOX9 pancreatic bud 19 47 PDX1, FOXA2, GATA4, NKX6.1, SOX9 PDX1, FOXA2, GATA4, NKX6.1, SOX9 "trunk" progenitor "tip" progenitor 23 8 weeks + PDX1, FOXA2, GATA4, NKX6.1, NKX2.2, ISL1 SOX9, FOXA2, PDX1 (weak) fetal beta cell ductal cell fetal 14 weeks GATA4 acini cell   Table data   Links: pancreas | exocrine pancreas | timeline

Spleen

Human Spleen Development

Embryonic Week	Carnegie Stage	Feature
Week 5	14	Carnegie stage 14 to 17 appears as a bulge in the dorsal mesogastrium. Mesothelium pseudostratified.
	15
Week 6	16	Mesothelium (pseudostratified} replaced with high columnar cells and then low columnar cells.
	17	Basement membrane present after this stage.
Week 7	18	Hematopoietic cells detected.
Week 8	20	Spleen is now apparent. Mesenchymal cells differentiated from cells in dorsal mesogastrium. Sinus and hilus formation after this stage.
	23	Arteries and veins parallel entries at this stage.
Fetal Week FA	Gestational Age GA	Feature
Week 15	Week 17	reticulum cells (alpha-smooth muscle actin-positive) scattered around the arterioles
Week 18-21	Week 20-23	reticulum cells increase in number and began to form a reticular framework. Accumulation of T and B lymphocytes within this framework, and a primitive white pulp was observed around the arterioles.
Week 22	Week 24	reticular framework antigenic diversity, and T and B lymphocytes were segregated in the framework. T lymphocytes were sorted into the alpha-SMA-positive reticular framework, and the periarteriolar lymphoid sheath (PALS) was formed around the arteriole. B lymphocytes aggregated in eccentric portions to the PALS and formed the lymph follicle (LF, reticular framework alpha-SMA-negative).
Week 24	Week 26	marginal zone appears in reticular framework around the white pulp.
Human embryonic data and fetal data    Links: spleen | Kyoto Collection | human timeline

Respiratory

Respiratory Stages - Species Comparison - Stages Gestational age (days)
Species	Term	Embryonic	Pseudoglandular	Canalicular	Saccular
human	280	< 42	52 - 112	112 - 168	168
primate	168	< 42	57 - 80	80 - 140	140
sheep	150	< 40	40 - 80	80 - 120	120
rabbit	32	< 18	21 - 24	24 - 27	27
rat	22	< 13	16 - 19	19 - 20	21
mouse	20	< 9	16	18	19
Data modified from    Links: respiratory | Respiratory Comparison | Mouse Human Respiratory | Mouse respiratory stages | mouse | rat | rabbit | Timeline Comparisons

Age (months)	Alveoli (million)	Respiratory Airways (million)	Generations of Airways
Birth	24	1.5
3	86	1.8
3	77	2.5	21
3	73	2.0
7	112	3.7
13	129	4.5	22
16	127	4.7
22	160	7.1
48	257	7.9
98	280	14.0	23
Adult	296	14.0	23
Data modified from   Links: respiratory | Respiratory Comparison | Mouse Human Respiratory | Mouse respiratory stages | mouse | rat | rabbit | Timeline Comparisons

Genital

Seminal Vesicle Timeline

Week	Fetal CRL mm	Event
13	80	seminal vesicles appear as lateral evaginations or out-pocketings from the lower portion of the mesonephric ducts (Wolffian ducts). At this stage their lateral diameter is always greater than the anteroposterior length.
14	100	sacculations or diverticula of the vesicles as evaginations of the walls of the vesicles. At this time 3 small but distinct sacculations may be counted at a given level in each vesicle.
19	170	dilatation of the mesonephric ducts in their lower portion, to form the ampullae of the deferent ducts. Sacculations of the ampullae in the form of slight but definite irregularities of their lumina. Each vesicle at this stage shows from 3 to 8 distinct sacculations at a given level. The lateral and antero—posterior diameters are now about equal.
21	180	sacculations of the mesonephric ducts have become well developed and the dilatation of the ducts to form the ampullae have assumed very nearly their proportions at birth.
25	220	vesicles and ampullae have assumed practically their adult form as regards their general topography and arrangement of sacculations. At a single level may be counted from~nine to twelve separate diverticula.
25 to 31	220 to 275	prostatic utricle opens into the posterior urethra. At the latter date has a bifid lumen showing the union of two partially fused tubes. At this stage 5 to 7 distinct sacculations in each vesicle at a given level.
Birth		vesicles show at a given level 7 distinct sacculations at one time. At this stage many of the diverticula are traversed by a network of fine trabeculae.
	Data   Links: seminal vesicle | timeline

Ovary

Human Ovary Timeline

Time	Carnegie Stage	Event
24 days	11	intermediate mesoderm, pronephros primordium
28 days	12	mesonephros and mesonephric duct
35 days	14	uteric bud, metanephros, urogenital ridge
42 days	17	cloacal divison, gonadal primordium (indifferent)
49 days	19	paramesonephric duct, gonadal differentiation
6-7 weeks (GA 8-9 weeks)		primordial germ cell mitosis proliferation commences
56 days	23	paramesonephric duct fusion (female)
12-14 weeks (GA 14-16 weeks)	fetal	primordial germ cell meiosis germ cell differentiation, formation of syncitial clusters of oogonia
15-18 weeks (GA 17–20 weeks)	fetal	breakdown of syncitial clusters and assembly of primordial follicles
Links: ovary | oocyte | timeline | Category:Timeline

Human Ovary Follicle Number Timeline

Bone

Shoulder Development Timeline

Carnegie Stage	Event
17	chondrogenic progenitor of the humerus and the medial border of the scapula can be observed.
18	chondrogenic progenitor for rest of the scapula appears.
19	glenohumeral joint will begin delaminating and showing a looser central band (interzone). Denser lateral bands will join the humeral head (caput humeri) and the margins of the articular surface of the scapula, thus forming the glenoid labrum (glenoid ligament).
21	long head of the biceps tendon present
22	glenoid labrum (glenoid ligament) present
23	coracohumeral ligament present
Week
Fetal Week 10	osteogenic process begins in the humeral head. Primitive glenohumeral ligament present
Fetal Week 11	osteogenic process begins in the scapula
Links: shoulder | joint | limb | timeline     Data from human histological study.

Human Pelvis Development

Carnegie Stage	Event
18	chondrification centres of the ilium, ischium, and pubis first appears. Located around the acetabulum and grew radially.
20	iliac crest formed while the iliac body's central part remained chondrified.
22	Sacroiliac joint forms.
22	Iliac body is discoid. The growth rate was greater in the ilium than in the sacrum-coccyx, pubis, and ischium.
23	Articulation of the pubic symphysis, connection of the articular column in the sacrum, and Y-shape connection of the three parts of the hip bones to the acetabulum.
Early Fetal	connection of the ischium and pubic ramus.
Data from human CT and MRI study.
Links: pelvis | pelvis timeline | joint | limb | timeline

Table Of Ossification Of The Bones Of The Superior Extremity

Bone	Centres	Time of appearance of centre	Union of primary and secondary centres; remarks.
Clavicle	Diaphysis	6th week	There are two centres in the shaft, a medial and a lateral. These blend on the 45th day (Mall). Shaft and epiphysis unite between the 20th and 25th years.
	Sternal epiphysis	18th to 20th year
Scapula	Primary centres:		The chief centre appears near the lateral angle. The subcoracoid centre appears at the base of the coracoid process and also gives rise to a part of the superior margin of the glenoid fossa. The coracoid process joins the body about the age of puberty. The acromial epiphysis centres (two or three in number) fuse with one another soon after their appearance and with the spine between the 22nd and 25th years (Quain); 20th year (Wilms). The subcoracoid and the epiphysis of the coracoid process, the glenoid fossa, the inferior angle, and the vertebral margin join between the 18th and 24th years in the order mentioned (Sappey).
	1. That of the body, the spine, and the base of the glenoid cavity.	8th week (Mall) 1
	2. Goraooid process	1st year
	3. Subcoracoid	10th to 12th year
	Epiphyses:
	Acromial epiphyses	15th to 18th year
	Epiphysis of the inferior angle.	16 to 18th year
	Epiphyses of the vertebral border.	18th to 20th year
	Epiphyses of upper surface of coracoid.	16th to 18th year.
	Epiphysis of surface of glenoid fossa.	16th to 18th year.
Humerus	Diaphysis	6th to 7th week (Mall)	The epiphyses of the head, the tuberculum majus and the tuberculum minus (the last is inconstant) unite with one another in 4th-6th year and with the shaft in 20th-25th year. The epiphyses of the capitulum, lateral epicondyle, and trochlea unite with one another and then in the 16th-17th year join the shaft. The epiphysis of the medial epicondyle joins the shaft in the 18th year.
	Epiphyses:
	Head	1st to 2d year
	Tuberculum majus	2d to 3d year
	Tuberculum minus	3d to 5th year
	Capitulum	2d to 3d year
	Epioondylus med	5th to 8th year
	Lateral margin of trochlea	11th to 12th year
	Epicondylus lat	12th to 14th year
Radius	Diaphysis	7th week (Mall)	The superior epiphysis and shaft unite between the 17th and 20th years. The inferior epiphysis and shaft about the 21st year (Pryor); M 21st year, F 21st-25th year (Sappey). Sometimes an epiphysis is found m the tuberosity (R. and K.) and in the styloid process (Sappey).
	Epiphyses:
	Carpal end	F 8th month - M 15th month (Pryor)
	Humeral end	6th-7th year
Ulna	Diaphysis	7th week	The centre for the shaft of the ulna arises a few days later than that for the radius. The proximal epiphysis is united to the shaft about the 17th year; the inferior epiphysis between the 18th and 20th years; F 20th - 21st years, M 21st - 24th years (Sappey). There is sometimes an epiphysis in the styloid process (Sohwegel) and in the tip of the olecranon process (Sappey).
	Epiphyses:
	Carpal end	F 6th-7th year - M 7th-8th year (Pryor)
	Humeral end	10th year
Carpus	Os capitatum	F 3d-6th month M 4th-10th month	The navicular sometimes has two centres of ossification (Serres. Rambaud and Renault). Serres and Pryor have described two centres of ossification in the lunatum. Debierre has described two centres in the pisiform, one in a girl of eleven, the other in a boy of twelve. The OS hamatum may have a special centre for the hamular process. Pryor has found two centres in the triquetrum. Pryor (1908), describes the centres of ossification of the carpal bones as assuming shapes characteristic of each bone at an early period.
	Os hamatum	F 5th-10th month M 6th-12th month
	Os triquetrum	F 2d-3d year M about 3 years
	Os lunatum	F 3rd-4th year M about 4 years
	Os naviculare	F at 4 years, or early in 5th year M about 5 years
	Os mult. maj.	F 4th-5th year M 5th-6th year
	Osmult. min.	F 4th-5th year M 6th-6th year
	Os pisiforme	F 9th-10th year M 12th-3th year
Metacarpals	Diaphyses	9th week (Mall)	The centres for the shafts of the second and third metacarpals are the first to appear. There may be a distal epiphysis for the first metacarpal and a proximal epiphysis for the second. Pryor (1906). found the distal epiphysis of the first metacarpal in about 6 per cent, of cases. It is a family characteristic. It arises before the 4th year and unites later. Pryor found the proximal epiphysis of the second metacarpal in six out of two hundred families. It unites with the shaft between the 4th and 6th-7th year; sometimes, however, not until the 14th year. In the seal and some other animals all the metacarpals have proximal and distal epiphyses (Quain). The epiphyses join the shafts between the 15th and 20th years. There may bean independent epiphysis for the styloid process of the 5th metacarpal. The epiphysis of the metacarpal of the index finger appears first. This is followed by those of the 3d, 4th, 5th, and 1st digits.
	Proximal epiphysis of the first metacarpal	3d year
	Distal epiphyses of the metacarpals	2d year
Phalanges	Diaphyses	9th week (Mall)
First row	Proximal epiphyses	1st-3rd year (Pryor)	The shafts of the phalanges of the second and third fingers are the first to show centres of ossification. The phalanges of the little finger are the last, the epiphysis in the middle finger is the first to appear. This is followed by those of the 4th, 2d, 5th, and 1st digits.
Middle row	Diaphyses	11th-12th week (Mall)	The centres in the shafts of this row are the last to appear. The epiphysis of the phalanx of the middle finger is the first to appear. This is followed by those of the ring, index, and little finger (Pryor).
	Proximal epiphyses	2nd-3rd year
Terminal row	Diaphyses	7th-8th week	The terminal phalanx of the thumb is the first to show a centre of ossification in the shaft. This is the first centre of ossification in the hand. It is developed in connective tissue while the centres of the other phalanges are developed in cartilage (Mall). The epiphysis of the ungual phalanx of the thumb is followed by those of the middle, ring, index, and little fingers. The fusion of the epiphyses of the phalanges with the diaphyses takes place in the 18th-20th year.
	Proximal epiphyses	2nd-3rd year
Sesamoid bones			Ossification begins generally in the 13th - 14th years, and may not take place until after middle life (Thilenius). For table of relative frequency in the embryo and adult see p. 385.
Days and weeks refer to the prenatal, years to the postnatal period. M = male F = female. According to Poirier, Traite d'Anatomie, p. 138, two centres appear in the eighth week, and unite in the third month to form a centre of ossification for the body of the scapula.
Links: limb | bone | upper limb ossification timeline | lower limb ossification timeline | Historic - Chapter 11 Development of the Skeleton | timeline | Category:Timeline     Table Data Reference

Table Of Ossification Of The Bones Of The Inferior Extremity

Bone	Centres	Time of appearance of centre	Time of fusion: general remarks
Os coxae	Os ilium	56th day (Mall)	The rami of the ischium and the pubis are united by bone in the 7th or 8th year (Quain) ( 12-14 year Sappey). In the acetabulum the three hip bones are separated by a Y-shaped cartilage until after puberty. In this cartilage between the ilium and pubis the "os acetabuli" appears between the ninth and twelfth years. This bone, variable in size, forms a greater or less part of the pubic portion of the articular cavity. Leche (1884). Krause (1885), and many others consider it primarily an independent bone. About puberty between the ilium and ischium and over the acetabular surfaces of these bones small irregular epiphyseal centres appear. The os acetabuli becomes imited to the pubic bone about puberty and soon afterwards the acetabular portions of the ilium and ischium and the ischium and pubis begin to become united by bone. The acetabular portions of the pubis and ilium are unite a little later. Osseous union takes place earlier on the pelvic than on the articular surface of the acetabulum. The union of the several primary centres and the epiphyses is usually completed about the twentieth year.
	Os ischii	105th day (Mall)
	Os pubis	4th to 5th fetal month
	Os acetabuli.	9th to 12th year
	Epiphyses: Those of the acetabulum	Soon after puberty
	Crest of ilium	Soon after puberty	Fuses with main bone 20th to 25th year
	Tuberosity of ischium	Soon after puberty	Fusion begins in the 17th year and is completed between the 20th and 24th years (Sappey)
	Ischial spine	Soon after puberty	18th to 20th year (Poirier).
	Ant. inf. spine of ilium	Soon after puberty	18th to 20th year (Poirier)
	Symphysis end of os pubis (1 or 2 centres)	18th to 20th year (Sappey)	After the 20th year
Femur	Diaphysis	43d day (Mall)
	Epiphyses: Distal end	Shortly before birth1	20th to 24th year
	Head	1st year	18th to 19th year
	Great trochanter	3d to 4th year (Osseous granules soon after birth, (Poirier)	18th year
	Small trochanter	13th to 14th year 8th year (Sappey)	17th year (Quain) Proximal epiphysis 18th to 22d year (Poirier)
Patella		3d to 5th year	The osseous patella reaches its definitive form soon before puberty
Tibia	Diaphysis	44th day (Mall)
	Epiphyses: Proximal end	About birth	19th to 24th year (Sappey)
	Distal end	2d year	16th to 19th year
	Tubercle (occas.)	13th year	Fuses with epiphysis of the proximal end and then with this to the diaphysis
Fibula	Diaphysis	55th day (Mall).
	Epiphyses: Distal end	2d year	20th to 22d year
	Proximal end	3d to 5th year	22d to 24th year
Calcaneus	Chief centre	6th fetal month	The chief nucleus is endochondral. A periosteal nucleus appears frequently in the 4-5 fetal month (Hasselwander)
	Epiphysis (distal end)	10th year (Quain) 7th-8th year ( Sappey)	15th-16th year (Quain) 16th-18th year (Poirier) M 17-21, average 20 years F 13-17, average 16 years (Hasselwander)
Talus		6th fetal month (Hasselwander)	In the 7th-8th year the posterior part of the talus, the os trigonum, is frequently ossified from a special centre (v. Bardeleben). It fuses about the 18th year.
Cuboid		About birth
Cuneiform III		1st year
Cuneiform I		2d-3d year
Cuneiform II		3d-4th year
Navicular		4th-5th year
Metatarsals	Diaphyses	8th-10th week	According to v. Bardeleben a second centre of ossincation appears much later than the primary in the navicular, and finally about the time of puberty a medial epiphyseal centre arises.
	Epiphyses	3d-8th year	The centre for the 2d metatarsal usually appears first, then come the 3rd, 4th, 1st and 5th. The epiphysis of the 1st metatarsal appears at the proximal end of the bone: the other epiphyses arise at the distal ends of the metatarsals. There may be a distal epiphysis in the first metatarsal also.2 In some instances a proximal epiphysis is formed on the tuberosity of the fifth metatarsal (Gruber). The epiphyses unite with the shafts in the 17-21 year in males and in the 14-19 year in females. (Hasselwander).
Phalanges:
Terminal row	Diaphyses	58th day (Mall)
	Epiphyses (distal)	4th year	M 13-23, average 16-21 year. F 13-17, average 14-17 year (Hasselwander).
Middle row	Diaphyses	4th-10th fetal month
	Epiphyses	3d year	M 15-19 year F 13-16 year (Hasselwander)
Proximal row	Diaphyses	3d fetal month
	Epiphyses	3d year	M 15-17 year. F 14-15 year (Hasselwander) The centres for the shafts of the phalanges often appear double, one for the dorsal and one for the plantar surface. The centres for the medial phalanges in each row usually appear before the more laterally placed centres. The centre for the 5th terminal phalanx appears much later than the other centres in this row (Mall). According to Rambaud and Renault the epiphyses arise each from two centres which fuse together. In the terminal phalanx of the great toe the ossification centre of the epiphysis often appears as early as the second or even the first year. (Hasselwander)
Sesamoid bones of the great toe		M 14th year F 12th-13th year	Ossification may begin in the 8th year in females, in the 11th in males (Hasselwander).
Poirier, Traite d'Anatomie, vol. 1. page 227, gives a summary of the literature on the time of the appearance of this epiphysis. The epiphysis has some medico-legal importance, since its presence or absence has been utilized to determine whether a child is born at term. Schwegel found it to appear between birth and the third year; Casper in the ninth fetal month. Hartmann found it lacking in 12 percent, of cases at birth and in 7 per cent, of cases present as early as the eighth fetal month. Mayet has described two centres of ossification for the proximal epiphysis of the first metatarsal, one of which represents the real metatarsal of the first digit.
Days and weeks refer to the prenatal, years to the postnatal period. M = male F = female. According to Poirier, Traite d'Anatomie, p. 138, two centres appear in the eighth week, and unite in the third month to form a centre of ossification for the body of the scapula.
Links: limb | bone | upper limb ossification timeline | lower limb ossification timeline | Historic - Chapter 11 Development of the Skeleton | timeline | Category:Timeline     Table Data Reference

Bone - Appearance and Fusion of Epiphyses Timeline
Before birth:
Both sexes	Appearance	Head of humerus, distal femur, proximal tibia, calcaneum, talus
Female	Appearance	Cuboid
During first year:
Both sexes	Appearance	Hamate, capitate, head of femur, third cuneiform
Female	Appearance	Capitulum, distal radius, distal tibia, distal fibula
Male	Appearance	Cuboid
During second year:
Both sexes	Appearance	Proximal phalanges of inner four fingers
Female	Appearance	First metacarpal, distal phalanges of thumb, middle and ring fingers, tarsal navicular, first and second cuneiforms
Male	Appearance	Capitulum, distal epiphysis of radius, distal fibula
At age of 2:
Both sexes	Appearance	Innerfourmetacarpals,firstmetatarsal,proximalphalangesoftoes, distal phalanx of hallux
Female	Appearance	Proximal phalanx of thumb, middle row of phalanges of fingers
Male	Appearance	First metacarpal, distal phalanx of thumb, and distal phalanx of index,firstcuneiform
At age of 3:
Female	Appearance	Patella, proximal fibula, second metatarsal, third metatarsal, middle phalangesof second, third and fourth toes, distal phalanges of third and fourth toes
Male	Appearance	Triquetrum, proximal phalanx of thumb, middle phalanges of middle and ring fingers, tarsal navicular, second cuneiform
At age of 4:
Both sexes	Appearance	Fourth metatarsal
Female	Appearance Fusion	Head of radius, fifth metatarsal Greater tubercle to head of humerus
Male	Appearance	Lunate, middle phalanges of index and litle fingers, distal phalanges of middle and ring fingers, second metatarsal, third metatarsal, middle phalanx of second toe
At age of 5:
Both sexes	Appearance	Navicular (carpal), multangulum majus, greater trochanter, distal phalanx of second toe
Female	Appearance	Medial epicondyle, distal ulna, lunate, triquetrum, multangulum minus, distal phalanx of index
Male	Appearance	Head of radius, distal phalanx of litle finger, patella, proximal fibula, fifth metatarsal, middle phalanges of third and fourth toes, distal phalanges of third and fourth toes
	Fusion	Greater tubercle to head of humerus
At age of 6:
Male	Appearance	Medial epicondyle, distal ulna, multangulum minus
At age of 7:
Female	Appearance	Distal phalanx of litle finger
	Fusion	Rami ofischium and pubis
At age of 8:
Both sexes	Appearance	Apophysis of calcaneus
Female	Appearance	Olecranon
At age of 9:
Female	Appearance	Trochlea, pisiform
Male	Fusion	Rami of ischium and pubis
At age of 10:
Male	Appearance	Trochlea, olecranon
At age of 11:
Female	Appearance	Lateral epicondyle
Male	Appearance	Pisiform
At age of 12:
Male	Appearance	Lateral epicondyle
At age of 13:
Female	Appearance	Proximal sesamoid of thumb
	Fusion	Lower conjoint epiphysis of humerus, distal phalanx of thumb, bodies ilium, ischium and pubis
Male	Fusion	Capitulum to trochlea and lateral epicondyle
At age of 14:
Female	Appearance	Acromion, iliac crest, lesser trochanter
	Fusion	Olecranon, upper radius, proximal phalanx of ring finger, distal phalanx of thumb, head of femur, greater trochanter, distal tibia and fibula, apophysis calcaneus, first metatarsal, proximal phalanges oftoes
Male	Appearance	Proximal sesamoid of thumb, base of fifth metatarsal
At age of 15:
Both sexes	Appearance	Sesamoid of little finger
	Fusion	Distal phalanges of second, third and fourth toes
Female	Appearance	Sesamoid of index and little fingers
	Fusion	Medial epicondyle, first metacarpal, proximal phalanx of thumb, distal phalanges of inner four fingers, proximal tibia, outer four metatarsals, middle phalanx of second toe, distal phalanges of innerfourtoes
Male	Appearance	Acromion
	Fusion	Ilium, ischium and pubis
At age of 16:
Female	Appearance	Distal sesamoid of thumb, tuber ischii
	Fusion	Inner four metacarpals, proximal phalanges of index, middle and litle fingers, middle phalanges of fingers
Male	Fusion	Lower conjoint epiphysis of humerus, medial epicondyle, olecranon, head of radius, distal phalanx of middle finger, apophysis of calcaneus
At age of 17:
Both sexes	Fusion	Acromion
Female	Fusion	Upper conjoint epiphysis of humerus, distal ulna, distal femur, proximal fibula
Male	Appearance	Distalsesamoidofthumb
	Fusion	First metacarpal, proximal phalanges of thumb and ring finger, middle phalanges, index, middle and ring fingers, distal phalanges of thumb, index, ring and little fingers, head of femur, greater trochanter,distal tibia and fibula, metatarsals, proximal phalanges of toes, middle phalanx of second toe, distal phalanx of hallux
At age of 18:
Female	Fusion	Distal radius
Male	Fusion	Inner four metacarpals, proximal phalanges of index, middle and litle fingers, middle phalanges of litle finger, proximal tibia
At age of 19:
Male	Appearance	Sesamoid of index, tuber ischii
	Fusion	upper conjoint epiphysis of humerus, distal radius and ulna, distal femur, proximal fibula
At age of 20:
Both sexes	Fusion	Iliac crest
Male	Fusion	Tuber ischii
At age of 21:
Both sexes	Appearance	Clavicle
Female	Fusion	Tuber ischii
At age of 22:
Both sexes	Fusion	Clavicle
Table Data: Flecker (1932) | Original table | bone timeline

Bone - Appearance and Fusion of Epiphyses Timeline

Bone - Appearance and Fusion of Epiphyses Timeline Before birth: Both sexes    Appearance     Head of humerus, distal femur, proximal tibia, calcaneum, talus Female   Appearance   Cuboid During first year: Both sexes   Appearance   Hamate, capitate, head of femur, third cuneiform Female   Appearance   Capitulum, distal radius, distal tibia, distal fibula Male   Appearance   Cuboid During second year: Both sexes   Appearance   Proximal phalanges of inner four fingers Female   Appearance   First metacarpal, distal phalanges of thumb, middle and ring fingers, tarsal navicular, first and second cuneiforms Male   Appearance   Capitulum, distal epiphysis of radius, distal fibula At age of 2: Both sexes   Appearance   Innerfourmetacarpals,firstmetatarsal,proximalphalangesoftoes, distal phalanx of hallux Female   Appearance   Proximal phalanx of thumb, middle row of phalanges of fingers Male   Appearance   First metacarpal, distal phalanx of thumb, and distal phalanx of index,firstcuneiform At age of 3: Female   Appearance   Patella, proximal fibula, second metatarsal, third metatarsal, middle phalangesof second, third and fourth toes, distal phalanges of third and fourth toes Male   Appearance   Triquetrum, proximal phalanx of thumb, middle phalanges of middle and ring fingers, tarsal navicular, second cuneiform At age of 4: Both sexes   Appearance   Fourth metatarsal Female   Appearance Fusion Head of radius, fifth metatarsal Greater tubercle to head of humerus Male   Appearance   Lunate, middle phalanges of index and litle fingers, distal phalanges of middle and ring fingers, second metatarsal, third metatarsal, middle phalanx of second toe At age of 5: Both sexes   Appearance   Navicular (carpal), multangulum majus, greater trochanter, distal phalanx of second toe Female   Appearance   Medial epicondyle, distal ulna, lunate, triquetrum, multangulum minus, distal phalanx of index Male   Appearance Head of radius, distal phalanx of litle finger, patella, proximal fibula, fifth metatarsal, middle phalanges of third and fourth toes, distal phalanges of third and fourth toes Fusion Greater tubercle to head of humerus At age of 6: Male   Appearance   Medial epicondyle, distal ulna, multangulum minus At age of 7: Female   Appearance   Distal phalanx of litle finger Fusion Rami ofischium and pubis At age of 8: Both sexes   Appearance   Apophysis of calcaneus Female   Appearance   Olecranon At age of 9: Female   Appearance   Trochlea, pisiform Male   Fusion   Rami of ischium and pubis At age of 10: Male   Appearance   Trochlea, olecranon At age of 11: Female   Appearance   Lateral epicondyle Male   Appearance   Pisiform At age of 12: Male   Appearance   Lateral epicondyle At age of 13: Female   Appearance   Proximal sesamoid of thumb Fusion   Lower conjoint epiphysis of humerus, distal phalanx of thumb, bodies ilium, ischium and pubis Male   Fusion   Capitulum to trochlea and lateral epicondyle At age of 14: Female   Appearance   Acromion, iliac crest, lesser trochanter Fusion   Olecranon, upper radius, proximal phalanx of ring finger, distal phalanx of thumb, head of femur, greater trochanter, distal tibia and fibula, apophysis calcaneus, first metatarsal, proximal phalanges oftoes Male   Appearance   Proximal sesamoid of thumb, base of fifth metatarsal At age of 15: Both sexes   Appearance   Sesamoid of little finger Fusion   Distal phalanges of second, third and fourth toes Female   Appearance   Sesamoid of index and little fingers Fusion   Medial epicondyle, first metacarpal, proximal phalanx of thumb, distal phalanges of inner four fingers, proximal tibia, outer four metatarsals, middle phalanx of second toe, distal phalanges of innerfourtoes Male   Appearance   Acromion Fusion   Ilium, ischium and pubis At age of 16: Female   Appearance   Distal sesamoid of thumb, tuber ischii Fusion   Inner four metacarpals, proximal phalanges of index, middle and litle fingers, middle phalanges of fingers Male   Fusion   Lower conjoint epiphysis of humerus, medial epicondyle, olecranon, head of radius, distal phalanx of middle finger, apophysis of calcaneus At age of 17: Both sexes   Fusion   Acromion Female   Fusion   Upper conjoint epiphysis of humerus, distal ulna, distal femur, proximal fibula Male   Appearance   Distalsesamoidofthumb Fusion   First metacarpal, proximal phalanges of thumb and ring finger, middle phalanges, index, middle and ring fingers, distal phalanges of thumb, index, ring and little fingers, head of femur, greater trochanter,distal tibia and fibula, metatarsals, proximal phalanges of toes, middle phalanx of second toe, distal phalanx of hallux At age of 18: Female   Fusion   Distal radius Male   Fusion   Inner four metacarpals, proximal phalanges of index, middle and litle fingers, middle phalanges of litle finger, proximal tibia At age of 19: Male   Appearance   Sesamoid of index, tuber ischii Fusion   upper conjoint epiphysis of humerus, distal radius and ulna, distal femur, proximal fibula At age of 20: Both sexes   Fusion   Iliac crest Male   Fusion   Tuber ischii At age of 21: Both sexes   Appearance   Clavicle Female   Fusion   Tuber ischii At age of 22: Both sexes   Fusion   Clavicle Table Data: Flecker (1932) | Original table | bone timeline

Muscle

Abdominal Wall Muscle Timeline

Carnegie Stage	Day	Mouse	Event
14	33	E10.5	mesoderm of primary body wall non-compact, coalesced in the ventral midline to create the abdominal cavity. Liver and stomach present. Dermomyotomes that are derived from somites have been formed.
16	40		Migration distance about 25% of the hemicircumference of the abdominal cavity. Lateral plate mesoderm has become more condensed and thicker in the area around the myoblasts. Primary abdominal wall ventral to this region was thinner and less dense. Suggests both myoblasts and connective tissue may migrate into the primary body wall or active cell proliferation.
17	42	E11.5	cells now migrated about 50% of the distance to the ventral midline. Inner and outer layers not yet not discernible.
18	44	E12.5	Separation of myoblasts into distinct inner and outer layers. Myoblasts in both inner and outer layers began to exhibit unidirectional orientation. Abdominal wall thicker (500 μm) in region where secondary structures forming compared with primary body wall region (260 μm). More dorsally positioned regions, outermost layer of connective tissue comprised approximately half of this thickness.
19	48		Segregation of the myoblasts into four distinct muscle groups with unidirectional orientation of myoblasts. Myoblasts migrated over half of the distance to the ventral midline. Abdominal wall remains thickest in the area where the muscles migrated and again the outermost layer of connective tissue comprises approximately half of the total thickness of the abdominal wall in this region. Primary abdominal wall that is ventral to the migrating myoblasts noticeably thinner. Human rectus completely separated after migrating over half the distance to the midline. In mouse, the rectus is not segregated from the other muscles until reaching the midline.
21	54	E14.5	Myoblasts have reached the ventral midline and myotubes were present and oriented uniformly within all muscle groups. Rectus abdominis formed distinct bundles of muscle indicating that development and differentiation of this muscle were more prominent in humans than in mice. Connective tissue layers form majority of the thickness of the abdominal wall, outermost layer of connective tissue majority of thickness.
23	58	E15.5	Rectus muscle forms 2 or 3 distinct layers and myotube orientation remained uniform in all muscles. External oblique and internal oblique expand in terms of thickness. Transversus remained a thin layer of muscle. Thickness of the connective tissue was reduced. The orientation of connective tissue layers in the obliques and transversus abdominis was dorsal to ventral.
Table Data

For additional muscle timelines see the page - Muscle Development Timeline

Adipose

Adipose Tissue Timeline

Anatomical Region	Specific Location	Start (weeks)	CRL (mm)	Complete (weeks)	CRL (mm)
Head	Buccal pad	14	100	17	153
	Cheek	14.5	103	17	150
	Chin	14.5	103	17	150
	Ocular pad	15	113	19.5	170
Neck	Neck	15	113	19.5	170
Thorax	Anterior wall	16	135	19.5	170
	Posterior wall	15	113	20.5	190
	Mammary	14.5	106	17.5	156
Abdomen	Abdominal wall	14.5	106	20.5	190
	Perirenal	15	113	20.5	190
Upper limb	Shoulder	15	113	23.5	216
	Forearm	16	131	20.5	190
	Arm	16	131	20.5	190
	Hand	16	131	l9.5	172
Lower limb	Gluteal	16	131	20.5	190
	Thigh	16.5	141	22.5	212
	Leg	16	131	22.5	212
	Foot	16	131	19.5	170
Table Notes - weeks are fertilization age (FA), not GA, both male and female data are combined. Table Data source   Links: adipose | Second Trimester

Nerve Endings

Comparison Human and Chick Neural Endings Timeline

Striped muscle and tendon	Sensory nerve endings	Motor nerve endings	Human (approximate age) weeks	Chick (Tello) (approximate age) days
Beginning differentiation	—	—	7	5
Myofibrils appear	Exploratory fibres with bulbous endings penetrate into muscles		8	6
Great increase of connective tissue. Vast numbers of nuclei	-	-	-	6-7
Isolated “fat” fibres	-	—	10-12	7
	Varicose endings in contact with myotubes	-	10	—
Tendon fibres diflerentiate	—	—	12	10
	Beginning pacinian corpuscle	—	20	11
Multiplication of myotubes	—	-	12-22	9-13
	Spindles differentiate	-	20	12
Dissociation of myotubes from one another. Nuclei become peripheral	Spindles become complex	—	22-26	13
	pacinian corpuscle well developed	—	26-28	15
		Plates begin to differentiate	26-28 (very rudimentary except tongue)	18
Data: Hewer (1935) (human weeks appear to be fertilization age based on CRL)   Links: timeline | chicken | skeletal muscle

Tongue

Fetal Tongue Circumference

Fertilisation Age (weeks)	Gestational Age GA (weeks)	Mean Circumference (mm)
12	14	28
13	15	33
14	16	36
15	17	37
16	18	43
17	19	48
18	20	51
19	21	55
20	22	58
21	23	62
22	24	64
23	25	70
24	26	73
Table data Tongue circumference measured by transvaginal ultrasonography between 14 and 17 GA weeks, and by abdominal ultrasound between 18 and 26 GA weeks of gestation. Links: tongue | fetal | ultrasound

Tooth

Human Dentition Timeline

Milk Dentition
Median incisors	6th to 8th month
Lateral incisors	8th to 12th month
First molars	12th to 16th month
Canines 1	7th to 20th month
Second molars	20th to 24th month
Permanent Dentition
First molars	7th year
Median incisors	8th year
Lateral incisors	9th year
First premolars	10th year
Second premolars	11th year
Canines	13th to 14th year
Second molars	13th to 14th year
Third molars	17th to 40th year
Approximate timings only, vary considerably according to racial, climatic, and nutritive conditions. Data source: Keibel and Mall (1912) The teeth     Links: tooth

Integumentary

Human Embryonic Eyelid Timeline

Carnegie Stage	Event
10	optic primordia appear
13	By the end of the fourth week the optic vesicle lies close to the surface ectoderm. The surface ectoderm overlying the optic vesicle, in response to this contact, has thickened to form the lense placode.
14	(about 32 days) the lens placode is indented by the lens pit.
15	(about 33 days) the lens pit is closed. The lens vesicle and optic cup lie close to the surface ectoderm and appear to press against the surface.
16	(37 days) Prior to the development of the eyelids, one small sulcus or groove forms above the eye (eyelid groove) and another below it.
17 - 19	grooves deepen, eyelid folds develop, first below, and then above, the eye.
19 - 22	eyelid folds develop into the eyelids and cover more of the eye as the palpebral fissure takes shape. The upper and the lower eyelids meet at the outer canthus in Stage 19.
20	the inner canthus is established.
23	closure of the eyelids is complete (Note - shown as still open in some Kyoto embryo).
	Data - from Kyoto embryos with Carnegie staging.   Links: eyelid

Glossary Links

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Cite this page: Hill, M.A. (2023, December 1) Embryology Timeline human development. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Timeline_human_development

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© Dr Mark Hill 2023, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G