The beginning of gastrulation is indicated by the formation of :

The formation of primitive streak from epiblast is an indicator that grastrulation has staed. Gastrulation is the process that establishes the three layers of germ cells in the embryo and occur during the third week of gestation. By the 15 th day of development , some epiblast cells lying along the central axis near the tail end of embryonic disc begin to proliferate. These proliferated cells form an elevation called primitive streak that bulges into amniotic cavity.