2. The Stages of Human
Embryonic Development
Human prenatal development is divided into
Human prenatal development is divided into
an embryonic period and a fetal period.
an embryonic period and a fetal period.
The embryonic period begins with fertilization
The embryonic weeks later.
and ends eight period begins with
The staging and ends embryos was later. The
fertilization of human eight weeks introduced
in 1914 by Franklin P. Mall at the Department of
staging of human embryos was introduced in
Embryology of the Carnegie Institution of
1914 by Franklin P. Mall at the Department of
Washington. Mall's sucessor, George L.
Embryology ofrefined the classification of of
Streeter, later the Carnegie Institution
human embryos into 23 stages, or
Washington.
"developmental horizons".
3. 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.
4. STAGE
1
Age: 1 Day
Size: 0.1 - 0.15 mm
dia.
Characteristic
Features:
Fertilization.
Includes penetrated
oocyte, ootid, &
zygote. Pronuclei
appear
5. STAGE 2
Age: 2-3
Days
Size: 0.1 -
0.2 mm
dia.
There are no
reconstructi
ons of Stage
2 embryos.
6. Age: 4-5 Days
STAGE 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.
7. Age: 6 Days
Size: 0.1 - 0.2 mm dia.
Characteristic Features: STAGE 4
Blastocyst is attaching
to endometrium
(beginning of
implantation).
Cytotrophoblast and
syncytiotrophoblast
become
distinguishable.
Amniotic ectoderm
develops.
8. STAGE 5a
Age: 7-8 Days
Size: 0.1 mm dia.
Characteristic
Features:
Solid
trophoblast, Am
niotic cavity;
implantation
9. 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.
10. 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
11. STAGE 8
Age: 23 Days
Size: 1-1.5 mm
dia.
Characteristic
Features: Embryo
nic disc is
piriform, Primitiv
e pit
appears, Neural
folds may begin to
form, Notochordal
and neurenteric
canals are
generally
detectable
12. STAGE 9
Age: 25 Days
Size: 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.
13. STAGE 10
Age: 28 Days
Size: 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
14. STAGE 11
Age: 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
15. STAGE 12
Age: 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
16. 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
17. Age: 33 Days
STAGE 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
18. 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.
19. Age: 38 Days Size: 8-11 mm
STAGE 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.
20. Age: 41Days
Size: 11-14 mm dia.
STAGE 17
Characteristic 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.
21. Age: 44 Days
STAGE 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.
22. 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.
23. Age: 49 Days Size: 18-22
STAGE 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.
24. 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.
25. Age: 53 Days
STAGE 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.
26. 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
