1. Gastrulation begins around day 14-15 with the formation of the primitive streak on the dorsal surface of the embryo, through which epiblast cells migrate inward to form the mesoderm and endoderm. 2. Cells invaginating the primitive pit move forward to form the notochordal process, which later forms the definitive notochord, a solid cord of cells. 3. By the end of the 4th week, the primitive streak begins to regress and disappear, and the embryonic disc becomes elongated with broad and narrow ends.

1. Trilaminar germ disc
By
DrManah Chandra Changmai MBBS MS

2. Gastrulation
Formation of embryonic mesoderm and endoderm
Primitive streak on the surface of the epiblast
The cephalic end of the streak,known as primitive node
Primitive node surrounds the a small primitive pit

4. Day 14 -15 raised groove appears
on dorsal surface of the epiblast –
cells migrate inward at the streak,
displacing underlying hypoblast to
become the endoderm
Day 16 – ingressing epiblast cells
form the mesoderm (a new layer
between the epiblast and
endoderm)
Epiblast cells that remain on the
embryo’s dorsal surface make up
the ectoderm
Ectoderm & endoderm = epithelia
Mesoderm = mesenchyme tissue
(mesen=middle; chyme=fluid)
Primitive streak

6. Cells invaginating the primitive pit Move straight
forward in cephalic direction upto prochordal plate.
Form a tube like process known as notochordal or
head process.
By the 18th
day,floor of the
notochordal process fuses with
underlying endoderm
Lumen of the notochordal
process disappears completely
Notocordal process forms a
narrow plate of cells
Notochordal cells proliferate
and forms solid cord,known as
definitive notochord.
Formation of notochord

9. Formation of cloacal membrane
Cloacal membrane is formed caudal to primitive streak,
consists of tightly adherent ectoderm and endoderm.
At 16th
day a diverticulum is formed at the posterior wall
of yolk sac which extends into connecting
stalk.
This diverticulum is called allantoenteric diverticulum or
allantois.
In humans it remains rudimentary.

11. Embryonic disc,initially flat and almost round
Later becomes elongated with broad cephalic
and narrow caudal end.
At the end of 4th
week primitive streak shows
regressive changes and soon disappears.
Sometimes remnants of primitive streak persists
persists and at birth cause tumours of sacrococcygeal
region.
Changes in embryonic disc

12. Further development of tropoblast
After 3rd
week mesodermal cells
penetrate the core of primary villi
forming secondary villi.
At the end of 3rd
week mesodermal cells
in the core of villi differentiate into blood
cells forming tertiary villus.

13. Tertiary villi

14. Cytotropoblastic cell penetrate into
the syncytium until they reach the
maternal endometrium
Establish contact with similar neighbouring
villous stems forming thin outer cytotropoblastic
shell.
Villi that extend from chorionic plate form decidua
basalis called the stem or anchoring villi.
Chorionic cavity becomes larger by 19th
or 20th
day
Embryo is attached to tropoblastic
shell by connecting stalk
Further development of tropoblast.

18. The end