Cleavage and Gastrulation
Eventually, one spermatozoon fuses with the ovum to r restore the diploid genomic condition and activates all the potentials in the fertilized egg cell or zygote to develop into a new individual of the next generation. But the zygote is one cell and the adult body in the Metazoa is constituted of many cells - from a few hundred to many billions of cells. It implies that the unicellular zygote must enter the phase of rapid divisions in quick succession to convert itself into a multicellular body.
Such a series of divisions of the zygote is known as cleavage or segmentation. In this multicellular structure formed as a result of cleavage the various cells or cell groups later become rearranged as layers and sub layers during a process called gastrulation. Cleavage and gastrulation are significant phases in the ontogenetic development because cleavage transforms the unicellular zygote into a multicellular body and gastrulation lays the foundation of primary organ rudiments so as to initiate the formation of organs according to the body plan of the particular metazoan group of animals to which the particular individual belongs.