The successful fusion of sperm and egg cells is known as fertilization. It forms the zygote that undergoes a series of cell division. During this process, blastocyst is formed four days after fertilization. This is simply the mammalian version of the blastula.
The human blastocyst
A. has three germ layers.
B. contains a fluid-filled central cavity.
C. is the expanded end of the gastrula.
D. is the fertilized egg.
E. gives rise to umbilical blood vessels.