The Stages of Human Embryonic Development Human prenatal development is divided intoHuman prenatal development is divided into an embryonic period and a fetal period.an embryonic period and a fetal period. The embryonic period begins with fertilizationThe embryonic weeks later. and ends eight period begins with The staging and ends embryos was later. Thefertilization of human eight weeks introduced in 1914 by Franklin P. Mall at the Department ofstaging of human embryos was introduced in Embryology of the Carnegie Institution of1914 by Franklin P. Mall at the Department of Washington. Malls sucessor, George L.Embryology ofrefined the classification of of Streeter, later the Carnegie Institution human embryos into 23 stages, orWashington. "developmental horizons".
The stages are based on a variety of morphological features and are independent of chronological age or size. As implied by the original term, "horizon", the stages are not definitive steps, that when combined lead to a fully formed human fetus, but rather they are a series of events that must be completed during development. As the description of each stage is based on the features seen in an "average" embryo individual embryos may not fit exactly into a particular stage.
STAGE 2Age: 2-3Days Size: 0.1 -0.2 mmdia. There are noreconstructions of Stage2 embryos.
Age: 4-5 DaysSTAGE 3 Size: 0.1 - 0.2 mm dia. Characteristic Features: Blastocy st develops and floats freely, "hatches" from zona pellucida. Inner cell mass and trophoblast. Embryonic disc consists of epiblast and hypoblast.
Age: 6 DaysSize: 0.1 - 0.2 mm dia.Characteristic Features: STAGE 4Blastocyst is attaching to endometrium (beginning of implantation). Cytotrophoblast and syncytiotrophoblast become distinguishable. Amniotic ectoderm develops.
STAGE 5a Age: 7-8 Days Size: 0.1 mm dia. Characteristic Features: Solid trophoblast, Am niotic cavity; implantation
Age: 17 Days Size: 0.2 mm dia. Characteristic STAGE 6 Features: Embryonic disc is usually elongated. Chorionic villi and intervillous space become distinct. Primitive streak and node, and prechordal plate appear; embryonic mesoblast begins to form. Secondary umbilical vesicle develops.
Age: 19 Days Size: 0.4 mm dia.STAGE 7 Characteristic Features: Embryonic disc is generally oval or piriform, Notochordal process becomes visible, Allantoic diverticulum becomes definite, Hematopoiesis begins
STAGE 8Age: 23 Days Size: 1-1.5 mmdia. CharacteristicFeatures: Embryonic disc ispiriform, Primitive pitappears, Neuralfolds may begin toform, Notochordaland neurentericcanals aregenerallydetectable
STAGE 9Age: 25 DaysSize: 1.5-2.5 mm dia.Characteristic Features: The embryo has the shape of the sole of a shoe, as seen dorsally. Mesencephalic flexure begins and otic disc forms. The embryo begins to be "lordotic". The neural groove is evident. The 3 major divisions of the brain are distinguishable and the heart begins to develop.
STAGE 10Age: 28 DaysSize: 2-3.5 mm dia.Characteristic Features: Fusion of the neural folds begins. The otic pit develops and pharyngeal arches 1&2 are visible on the surface. Optic, thyroid, & respiratory primordia begin to develop. Cardiac loop begins to appear and intermediate mesoderm becomes visible
STAGE 11Age: 29 Days Size: 2.5-4.5 mm dia. Characteristic Features: The rostral neuropore closes and the otic pit is still open. The optic vesicles develop and sinus venosus begins. The mesonephric duct & tubules appear
STAGE 12Age: 30 Days Size: 3-5 mm dia. Characteristic Features: The caudal neuropore closes. Four pharyngeal arches are visible and upper limb buds are appearing. Secondary neurulation commences. Lung bud appears. Cystic primordium & dorsal pancreas becomes distinguishable
Age: 32 Days Size: 4-6 mm dia. STAGE 13 Characteristic Features: Otic vesicle is closed. The lens disc is usually not yet indented and four limb buds are usually visible. Retinal & lens disc develop. Septum primum & foramen primum are distinct in heart. The right and left lung buds are recognizable
Age: 33 DaysSTAGE 14 Size: 5-7 mm dia. Characteristic Features: Lens pit appears. Endolymphatic appendage becomes defined. Upper limb buds are elongated & tapering. Optic cup develops. Adenohypophysial pouch is defined. Ureteric bud appears
Age: 36 Days Size: 7-9 mm dia. STAGE 15 Characteristic Features: Lens pit is closed, nasal pit is appearing, and hand plate is forming. Future cerebral hemispheres become defined and retinal pigment becomes visible. Lobar buds appear in bronchial tree.
Age: 38 Days Size: 8-11 mmSTAGE 16 dia. Characteristic Features: Retinal pigment is visible in the intact embryo. Nasal sacs face ventrally. Foot plate appears. Epiphysis cerebri develops. Neurohypophysial evagination is visible. Lobar bronchi are evident.
Age: 41Days Size: 11-14 mm dia. STAGE 17Characteristic Features: Head is relatively larger and trunk is straighter. Nasofrontal groove and auricular hillocks are distinct. Finger rays become visible. Chondrification begins in humerus, radius, and some vertebral centra. Segmental bronchial buds develop. Vermiform appendix becomes visible.
Age: 44 DaysSTAGE 18 Size: 13-17 mm dia. Characteristic Features: The body is more cuboidal. Digital plate of hand is notochord. Toe rays begin to appear. Oronasal membrane develops. 1 to 3 semicircular ducts are present in internal ear. Septum secundum & foramen ovale are distinct in heart.
Age: 46 Days Size: 16-18 mm dia. STAGE 19 Characteristic Features: The trunk is elongated and straightening. Limbs extend nearly directly forward. Toe rays are prominent, but interdigital notches have not yet appeared. Olfactory bulb developes. Cartilaginous otic capsule is visible. Posterior epithelium of cornea begins to develop.
Age: 49 Days Size: 18-22STAGE 20 mm dia. Characteristic Features: Upper limbs are longer & are bent at elbows. Nerve fibers reach optic chiasma. S- shaped renal vesicles are visible in metanephros.
Age: 51 Days STAGE 21 Size: 22-24 mm dia. Characteristic Features: Hands approach each other, fingers are longer, and feet approach each other. Cortical plate becomes visible in brain. Substania propria of cornea develops. Glomerular capsules develop in metanephros.
Age: 53 DaysSTAGE 22 Size: 23-28 mm dia. Characteristic Features: Eyelids and external ears are better developed. Adenohypophysial stalk is now incomplete. Scleral condensation is visible. Some large glomeruli are present in metanephros.
Age: 56 Days Size: 27-31 mm dia. STAGE 23 Characteristic Features: Head is more rounded. Limbs are longer and better developed. Humerus presents all cartilaginous phases. Bone collar of humerus has not yet been eroded through completely. Secretory tubules of metanephros become convoluted, and numerous large glomeruli are present