When the developing animal reaches the 32 cell stage it is called a morula. When the animal reaches 500 - 2000 cells it is called a blastula if the animal consists of a hollow ball of cells, or a blastodisc. A cavity called a blastocoel is present at this stage. Amphibians and most fish form blastulas. Reptiles, birds, monotremes and some fish form blastodiscs.
Placentals form a blastodisc similar to reptiles.
But, instead of the disc sitting on sac of yolk, it lies inside a hollow ball of cells called a trophoblast. From these three tissue layers, all the organ systems of the body develop. In sea urchins, one side of the blastula invaginates. The blastula collapses from a hollow ball to a two cell layered cup-shaped structure. The blastocoel disappears.