G1--- the cell is growing up feeding for 1
S-- the cell starts to repucate the chromosomes
G2--- the cell is growing for 2
<span> After fertilization, the </span>zygote<span> undergoes cleavage to form the blastula. The blastula, which in some species is a hollow ball of cells, undergoes a process called gastrulation, in which the three germ layers form.</span>