a) Some cells of the inner cell mass diffrentiate into flattened cells that to line its free surface. These constitute the ENDODERM- the FIRST GERM LAYER to be formed.

b) The remaining cells of inner cell mass become columnar cells from the SECOND GERM LAYER- ECTODERM. The embryo is in the form of a disc with 2 layers.

c) A space appears between the ectoderm and the trophoblast, this is the “AMNIOTIC CAVITY” which is filled by amniotic fluid or “LIQUOR AMNEU”. The roof of this cavity is formed by amniogenic cells derived from trophoblast and the floor from ectoderm.

d) “PRIMARY YOLK SAC” a cavity is developed from the flattened cells arising from the endoderm or from trophoblast which spread and line the inside of the blastocystic cavity by the cells of endodermal origin.

Soon after the formation of the prochardal plate some of the ectodermal cells lying along the central axis near the tail end of the disc begin to proliferate and form an elevation that bulges into the amniotic cavity. This elevation is called primitive streak. The primitive streak is at first a rounded or oval swelling, but with elongation of embryonic disc it becomes a linear structure lying in the central axis of the disc.

The intraenbryonic mesoderm spreads through out the disc except in the region of prochardal plate. The mesoderm extends on cranial to the prochordal plate, and here mesoderm from the two sides becomes continous across the midline. In the region of the prochordalplate the ectoderm and endoderm remain in contact. In later development the ectoderm and endoderm mostly persists as a living epithelium. On the other hand the bulk of the tissues of the body is formed predominantly from mesoderm. As there is no mesoderm in the prochardal plate, this region remains relatively thin and later forms ” BUCCOPHARNGEAL MEMBRANE”.

The prmitive streak gradually elongates along the central axis of the embryonic disc. The disc also elongates and becomes pear shaped.