Hint: Embryo is said as the starting of the next generation. It is developed as a result of fusion between male and female gametes. After fertilization implantation takes place in the female uterus the structure is called an embryo. Embryo takes nutrition from a structure known as placenta.
During the time of embryo development many events occur. One such stage is cleavage. First cleavage is completed and processed after 30 hours of fertilization. Cleavage furrow passes from animal- vegetal axis as well as centre of zygote (Meridional plane).
It divides and breaks the zygote completely into two blastomeres (Holoblastic cleavage). Second cleavage is completed and processed after 60 hours of fertilization. It is also meridional but occurs at the right angle to the first one.
Morula :- Cleavage usually leads to development of a solid ball of cells, Morula constitutes 8-16 cells. Zona pellucida now also forms the outer cover. Morula now undergoes compaction that is the outer/peripheral cells are smaller with tight junctions while the inner cell mass consists of slightly large, rounded cells with gap junctions. After compaction morula forms gastrula.
Gastrula :- the gastrulation is defined as movement of cells in small masses or sheets so as to form primary germinal layers. There are three types of primary germinal layers - endoderm, ectoderm and mesoderm. The cell movements that take place during gastrulation are called morphogenetic movements since they lead to initiation of morphogenesis; the product of gastrulation is called gastrula.
Note: All cleavage divisions are generally mitotic and resultant daughter cells are termed as blastomeres. During the process of cleavage, there is no growth in the resulting blastomeres and the total size and volume of the embryo remains the same, because in cleavage divisions interphase is without growth phase.