The first indication of gastrulation in the embryo is
The formation of the primitive streak on the dorsal surface of the bilaminar embryonic disk formed from proliferation at the end of 2nd week (day 14) is the first indication of gastrulation.
Formation of the primitive streak
Formation of the notochord
Formation of the neural tube
Formation of extraembryonic mesoderm
Development period- week 1,2,3,4