Cleavage and Blastocyst

Even though the cleavage is complete and all the blastomeres are of more or less equivalent size. Synchronization of mitoses in the blastomeres is lost very early. Even the first two blastomeres may cleave at dissimilar rates; consequently, a 3 cell stage and subsequently stages of five, six, seven blastomeres and so forth are found. The result of the cleavage is a solid mass of cells a morula, where some cells are superficial and others lie inside, completely cut off from the surface through them enveloping cells.

In due course the superficial cells join to make a distinct epithelial layer. This layer provides rise to most of the extra embryonic parts (the embryonic membranes), serves to attach the embryo to the uterine wall, and mediates in the supply of nourishment to the embryo from the maternal body via the placenta. This outer layer of the mammalian embryo is termed as the trophoblast (the term trophe meaning nourishment). The cells lying in the interior are known as the inner cell mass (ICM), and it is these cells that provide material for the formation of the embryo proper. Hence they may, be referred to as the formative cells. Sooner or later a cavity appears inside the compact mass of cells of the morula. The cavity is formed of crevice that appears among the inner cell mass and the cells of the trophoblast. Fluid is imbided into this cavity, so that it enlarges. The trophoblast becomes lifted off most of the inner cell mass remaining attached to it on one side only. This side corresponds late to the dorsal side of the embryo. A mammalian embryo at this stage is termed as a blastocyst.