 One more cell division brings the number of cells to 16, at which time it is called a morula, instead of a zygote.This ball of 16 cells then reorganizes into a hollow sphere called a blastocyst.As the number of cells grows from 16 to between 40 and 150, the blastocyst differentiates into two layers, an outer sphere of cells called the trophoblast and an inner cell mass called the embryoblast.