During the first four days after conception, the zygote undergoes rapid cell divisions and becomes multicellular. What is this cell division called?

The correct asnwer is blastulation. It occurs during first four days after conception. Blastula is formed from morula, and a cavity called bastocoel is formed n the centre of blastomeres. It is called as a blastocyst in mammals. The layer which covers it is called as blastoderm