The umbilical cord in mammals contains:

All extra-embryonic membranes:

$A$ : 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.

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

When does a foetus develop limbs and digits?

Which of the following participate in the formation of the placenta?