Phases of embryonic development undergoes a long series of changes. It is divisible into the following stages.
(i) Gametogenesis
(ii) Fertilization
[a] Spermatogenesis
[b] Oogenesis
(iii) Cleavage
(iv) Implantation
(v) Gastulation
(vi) Organogenesis
Therefore, the third phase in the development of mammal is cleavage.
It includes the rapid mitotic division of the zygote to form a single layered hollow spherical larva called blastula and its formation is called blastulation.