Gastrulation:
During implantation, zona pellucida disappears and the trophoblast cells become giant-sized and aggressively invade the uterine tissue and make contact informing the placenta.
Inner cell mass at this time undergoes differentiation and a layer of very flat cells appear on the interior surface of the inner cell mass that forms future endoderm. The remaining dorsal inner cell mass is called epiblast, which is presumptive material for ectoderm and mesoderm.
Formation of three germinal layers from a single-layered blastocyst. The blastocoel gets obliterated & a new cavity called gastric or archenteron is formed. It is called a gastrula. The method by which blastula becomes or converted into gastrula is called Gastrulation.
The mesodermal cells soon involute through the primitive streak and diverge anteriorly and laterally, occupying a position between ectoderm and endoderm. Epiblast also contributes some cells to endoderm. The cells migrating to their future places by amoeboid movements.