Trilaminar germ disc
DrManah Chandra Changmai MBBS MS
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
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
Epiblast cells that remain on the
embryo’s dorsal surface make up
Ectoderm & endoderm = epithelia
Mesoderm = mesenchyme tissue
Cells invaginating the primitive pit Move straight
forward in cephalic direction upto prochordal plate.
Form a tube like process known as notochordal or
By the 18th
day,floor of the
notochordal process fuses with
Lumen of the notochordal
process disappears completely
Notocordal process forms a
narrow plate of cells
Notochordal cells proliferate
and forms solid cord,known as
Formation of notochord
Formation of cloacal membrane
Cloacal membrane is formed caudal to primitive streak,
consists of tightly adherent ectoderm and endoderm.
day a diverticulum is formed at the posterior wall
of yolk sac which extends into connecting
This diverticulum is called allantoenteric diverticulum or
In humans it remains rudimentary.
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
Changes in embryonic disc
Further development of tropoblast
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.
Cytotropoblastic cell penetrate into
the syncytium until they reach the
Establish contact with similar neighbouring
villous stems forming thin outer cytotropoblastic
Villi that extend from chorionic plate form decidua
basalis called the stem or anchoring villi.
Chorionic cavity becomes larger by 19th
Embryo is attached to tropoblastic
shell by connecting stalk
Further development of tropoblast.