During the development of an embryo,which of the following occurs first?

The mammalian primitive streak gives rise to

In which stage of development do the embryonic cells form the germinal layers through movement?

All extra-embryonic membranes:

The third phase in the development of a mammal is

$A$ : The morula passes through the phase of compaction,producing two major types of cells: the peripheral cells and the inner cell mass.
$R$ : The descendants of the inner cell mass become the trophoblast cells.