Embryonic Period ( 3rd to 8th weeks)
Fetus (3rd month to Birth)
First week of development:
Ovulation to Implantation
Second week of development:
Bilaminar germ disc
Two embryonic layers – epiblast and hypoblast.
Two cavities – Amnionic cavity and
exocoelomic cavity (yolk sac).
Two layers of trophoblast – cytotrophoblast &
Formation of extraembryonic mesoderm.
Third week of development:
Trilaminar germ disc
Formation of the Three Germ Layer - GASTRULATION
- Migration of epiblast cells to form the mesoderm and endoderm
- The remaining epiblast gives rise to the ectoderm
Formation of the primitive streak, pit and node
Formation of the notochord
Embryonic Period/ Organogenesis (3rd to 8th weeks)
Formation & development of body organs from embryonic tissue is
Each of the 3 germ layers gives rise to a number of specific tissues &
Formation of the Neural tube (Neurulation)
The neuro-ectoderm (neural plate) is derived from the ectoderm
overlying the notochord.
By the end of third week the lateral edges of neural plate elevates to
form neural fold.
Depressed mid region forms the neural groove
The neural groove deepens and eventually forms a neural tube.
Two masses of ectoderm at edges of neural plate, form neural crest.
Initially the neural crest separates neuro-ectoderm from skin
As folding of the neural tube occurs, the neural crest cells detach from
the ectoderm and form clusters that migrate into the mesoderm.
Derivaties of Ectoderm
Mucous membrane of lips ,cheeks , gums, part of floor of
mouth , part of the palate, nasal cavity & sinuses
Lower part of anal canal ( below pectinate line)
Terminal parts of male urethra
Outer surface of labia minora & whole of labia majora
Anterior epithelium of cornea, epithelium of conjunctiva,
epithelial layers of ciliary body & iris
Outer layer of tympanic membrane, epithelial lining of
membranous labyrinth including the special end organs
Lacrimal canaliculi, sac, nasolacrimal duct
B. Glands :
• Sweat glands, sebaceous glands, parotid gland, mammary
anterior of pituitary gland
C. Other derivatives:
• Tooth enamel
A. Neural tube
Retina & Optic nerve & musculature of iris
Pineal & pituitary gland
Neuroglia – Ependymal cells , Macroglia(Astrocytes, oligodendrocytes)
B. Neural crest
Chromaffin cells (adrenal medulla)
Dorsal root ganglia & dorsal root of spinal nerve
Sensory ganglia of V, VII, IX & X cranial nerves
Melanocytes of skin
Bones & connective tissues of cranio-fascial structues
Parts of heart
3. Ectodermal Placode
Otic placode – gives rise to structures needed for hearing &
Lens placode – forms lens
Nasal placode – nasal cavities & para nasal sinus
Derivaties of Mesoderm
Mesoderm in the lateral part of the embryo is divided into three distinct
- a longitudinal column of cells that lies next to the notochord
- it gives rise to the axial skeleton and skeletal muscle
- it gives rise to the genitourinary system
Lateral plate mesoderm
- gives rise to body wall structures
- is continuous with the extra-embryonic mesoderm
- splits into two layers enclosing the intra-embryonic coelom
a. Somatic layer of mesoderm – mesodermal derivatives of the body wall.
b. Splanchnic layer of mesoderm – wall of the gut tube & connective tissue of
The Paraxial mesoderm further forms segmental cuboidal
bodies by third week - somites
Differentiates – sclerotome & dermatomyotome.
It gives rise to the axial skeletal ,the associated muscles
( true back muscles) and part of the overlying dermis.
There are about 42- 44 pairs of somites – but a first
occipital and last 5-7 coccygeal somites disappear.
Lateral plate mesoderm & intraembryonic coelom
A cavity – intraembryonic coelom appears in the lateral plate
Has somatic (parietal) layer and splanchnic (visceral) layer:
a. Somatic layer of mesoderm – mesodermal derivatives of
the body wall.
b. Splanchnic layer of mesoderm – wall of the gut tube &
connective tissue of viscera.
Development of blood vessel
Blood cells and blood vessels develop from the mesoderm.
Mesodermal cells differentiate into endothelial cells surrounding a
central group of erythroblasts. These are blood islands that coalesce to
form blood vessels. Almost all parts of the mesoderm gives rise to blood
Differentiation of blood vessels
Blood islands and eventually blood vessels appear:
in the extra-embryonic mesoderm in the second week
in the intra-embryonic mesoderm in the third week
the primitve heart tube develops in the cardiogenic mesoderm (in the
transverse mesoderm) at the beginning of the fourth week and a
primitive circulation is established
Derivatives of Endoderm
Epithelium : G.I.tract, liver, pancreas, gallbladder,
part of the urinary bladder & urachus.
Epithelium: trachea, bronchi & lungs.
Epithelium [in part] : pharynx, thyroid, tympanic
cavity, auditory tube & tonsils.
Folding of the Embryo
Folding occurs by differential growth of tissues. Neural ectoderm
grows faster than the surrounding skin ectoderm and consequently fold
to form a neural tube. Similarly, skin ectoderm grows faster than the
underlying mesoderm and endoderm, and this differential growth
causes folding of the trialminar disc and gives shape to the embryo.
Folding occurs mainly at the edges of the embryonic disc and forms
three main folds:
Lateral folds - convert the embryo into a tubular structure.
The notochord, neural tube and somites stiffen the dorsal axis of the
As a result of the formation of the head fold
The foregut is formed by folding of the endoderm
The stomadeum is an invagination of ectoderm, and has the
buccopharyngeal membrane separating it from the foregut. It opens
into the amniotic cavity.
The pericardial cavity and cardiogenic mesoderm are shifted to the
ventral aspect of the embryo and lie ventral to the foregut.
The part of the transverse mesoderm between the pericardial cavity
and the yolk sac is the septum transversum proper. In it the liver
The amniotic cavity extends ventral to the cranial end of the
The yolk sac is constricted from the cranial aspect.
As a result of the formation of the tail fold
The hindgut is formed
The cloaca is an invagination of ectoderm and has the cloacal
membrane separating it from the hindgut.
The connecting stalk is shifted ventrally
The allantoic diverticulum is shifted ventrally. It is an invagination of
hindgut endoderm into the connecting stalk.
The amniotic cavity extends ventral to the caudal end of the embryo.
The yolk sac is constricted from the caudal end
Transverse folding of the embryo
Converts the endoderm into a primitive gut tube
The intra-embryonic coelom surrounds the gut tube
The communication between the intra- and extra- embryonic coeloms
becomes constricted and eventually obliterated
Note that drastic and important changes occur in the
embryonic cavities as a consequence of folding:
The amniotic cavity surrounds the embryo completely on all aspects
and becomes the predominant cavity. It enlarges progressively.
The yolk sac becomes constricted on all sides, and becomes a small sac
connected to the midgut by a narrow vitelline duct. It becomes
The extra-embryonic coelom is gradually obliterated by the expanding
amnion and eventually disappears completely
Fetus (3rd month to Birth)
The Period from the beginning of 3rd month to the birth is known as
The main character of this period is the period of rapid growth and
maturation of the tissue.
During the 4th week of the development, the age of the embryo is
expressed in the number of somites.
The length of the embryo between 4th and 8th developmental weeks
is indicated as the crown-rump length (CRL) and is expressed in
During fetal development (9th - 40th week) the length of the
conceptus is indicated as CRL or as the crown-heel length (CHL)
expressed in centimeters.
Growth in length is most intensive during the 3rd to the 5th month,
while the increase in weight is most evident during the last two
months of gestation.
Monthly changes: 9th – 12th week
The head constitute half of fetus.
Erythropoiesis occurs in liver.
Metanephric kidney becomes functional and urine is formed.
Physiological hernia occurs and persists till 10th week & returns by 11th
Primary Ossification centre appears in the long bones.
External genitalia differentiated, identification of sex possible.
Head growth slows down but rapid growth in length. The not so
developed lower limbs start to develop.
Ossification centres becomes clearly evident in x-rays.
Erythropoiesis begins in spleen. In liver it is seized.
Primary follicles in ovaries seen.
Scalp hair pattern determined.
Soft thin hairs ‘lanugo’ begins to develop.
Growth slows down.
Quickening felt by mother i.e., first sign of locomotion.
Development of uterus is complete.
Testes begin to descend.
Pneumocytes type II is differentiated and becomes functional.
Therefore, surfactants are released.
Formation of capillaries plexus and pink coloration of the skin seen.
Finger nails visible.
26th- 29th Weeks
Bone marrow differentiated and becomes functional.
Erythropoiesis ceases in spleen.
Lungs have matured enough to be functional. Rhythmic breathing is
possible and the foetus is viable.
Subcutaneous fat develops- smoothing the wrinkles.
Hairs develop on the scalp.
Toe nail visible.
30th to 34th weeks
Quantity of fat increases considerably.
Pupillary light reflex develops by 30th weeks.
All body systems develop by 32nd weeks and foetus can be born
35th- 38th Weeks
Fat increase considerably.
The skin is covered by a whitish, fatty substance (vernix caseosa)
composed of secretory products from sebaceous gland.
Head circumference is the largest of all body parts
Time of birth
280 days or 40 weeks after the onset of last normal menstrual
More accurately, 266 days or 38 weeks after fertilization.
Preterm : born before 37 weeks of gestation
Post term or post maturity : born after 42 weeks of gestation