How is  morula formed?

Cleavage is holoblastic type but there is tendency to show size differences of the blastomeres from the very start. Cleavage occurs during its passage through fallopian tube to uterus.

The first cleavage takes place in the cytoplasm of ovum on an imaginary axis (Animal pole to vegetal pole). The first division results into two blastomeres. One blastomere is slightly larger than the other. The larger cell divides first and thus forms the three blastomeres. Then the smaller blastomere divides and thus four blastomeres are formed. It is 4-celled stage.

2nd cleavage is at right angle to the 1st cleavage. One member of the large blastomeres of 4-cell stage divides forming 5-cell condition followed 6-7 stage ultimately 8 celled stage. Successive cleavage divisions result in the formation of solid mass of cells. Thus solid ball-like stage is called morula stage.

 Cleavage in mammalian ovum (pig) in sectional view
         A. zygote; B. two-cell stage;C. four-cell stage;
         D. sixteen-cell stage E. morula stage   F and G. Blastocyst 

Conversion of morula into blastula is initiated by dynamic rearrangement of small blastomeres. A central fluid filled cavity called blastocoel (segmentation cavity) appears. Externally placed blastomeres lose their rounded form and become flat. This trophoblast along with lining of uterus forms extraembryonic membranes. This provides protection and nourishment to the developing embryo.

Simultaneously zona pellucida disappears and embryo gets implanted to the uterus. Below the blastodermic vesicle there appears formative tissue i.e. inner cell mass. From this inner cell mass, proper embryo is formed.