CLEAVAGE

 The human egg is alecithal because it is contain a negligible or no yolk .

Thus the cleavage division in human is holoblastic i. e. it divides the zygote completely into daughter cells or blastomeres .

It is a series of mitotic division of zygote ,resulting increase in the number of cell .

These cells which becomes smaller with each cleavage divisions are know as blastomeres .

The first cleavage division occurs approximately 30 hours after fertilization to form two cell stage .

Approximately 40 hours after fertilization four cell stage occurs .And three days after fertilization 12-16 cell stage are formed .

Until the eight cell stage they form a loosely arranged clamp. After the third cleavage however blastomeres maximized their contact with each other forming a compact ball of cells held together by tight junction this process is called compaction .

The process compaction segregates inner cells which communicate extensively by gap junction from outer cells .

Approximately days after fertilization cell of the compacted embryo divided again to forma 16 cells morula .

The inner cell of the morula constitute the inner cell mass and surrounding cell compose the outer cell mass .

The inner cell mass gives rise to tissues of the embryo proper and the outer cell mass forms the  trophoblast which later contributes to form the placenta .