Blastulation is followed by gastrulation.
Gastrulation - The blastocyst continues with the development process even when implantation is taking place. Formation of gastrula from blastula is known as gastrulation.
Gastrulation is process during which primary germinal layers namely ectoderm, mesoderm and endoderm are formed by morphogenetic movement and rearrangement of the cells.
Gastrulation and implantation of blastocyst takes place simultaneously.
Gastrulation involves following changes :
1) Formation of endoderm : Some cells covering the free surface of the embryonal knob (exposed to the cavity of blastocytes) become flat and form first endodermal cells. Then, they rapidly multiply, spread out in all directions and soon form a complete lining inside the trophoblast of the blastodermic. vesicle. This lining constitutes the endoderm. The endodermal tube so formed is primitive gut or archenteron. Its cavity also called gastrocoel is simply the earlier blastocoel with a changed endodermal lining. The endoderm located under the embryonal knob is embryonic endoderm. The remaining endoderm along with trophoblast forms the primary yolk sac. It encloses a fluid and not yolk.
2) Formation of embryonic disc : The blastocyst continues to grow in size. With the growth of blastocyst, after the formation of endoderm, the embryonic knob becomes columnar to form embryonic disc.
3) Formation of amniotic cavity : The space between embryonic disc and the trophoblast is called amniotic cavity. It is filled with amniotic fluid. The roof of this cavity is formed by amniogenic cells derived from trophoblasts and its floor is made up of embryonic disc.
4) Formation of ectoderm : After the formation of mesoderm the remaining cells of embryonic disc get arranged in a layer called ectoderm.
5) Formation of embryonic mesoderm : At the hind end of the embryonic disc, the cells start proliferating at an increased rate. Addition of cells causes localized thickness of embryonic disc. The proliferated cells later detach from embryonic cells and spread between trophobalst and endoderm. The new layer of cells so formed, is mesoderm, which separates the yolk sac and the amniotic cavity from the trophoblast.
6) Formation of extra-embryonic coelom : A large cavity called extra embryonic coelom is formed in the extra embryonic mesoderm. Extra embryonic mesoderm splits this cavity into layers. -
A) Outer mesoderm lining the trophoblast is called parietal extra embryonic mesoderm.
B) Inner mesoderm covering the primary yolk sac and wall of amniotic cavity is termed the visceral extra embryonic mesoderm.
7) Formation of chorion and amnion : Chorion consists of trophoblasts lined by parietal extra embryonic mesoderm. Amnion consists of amniogenic cells covered by visceral extra embryonic mesoderm.