Blastulation

Blastulation : It is the process by which the solid morula transforms into the hollow, multicellular blastocyst. The embryo (now a blastocyst) that enters the uterus remains floating freely in the uterine cavity for 2–4 days after entry — that is, until about the end of the 7th day after fertilization. The outer layer of cells seen in the morula now becomes a distinct layer, the trophoblast; cells of the trophoblast begin absorbing the glycogen-rich uterine milk, and the blastocyst doubles in size, from about 0.15 mm to 0.30 mm, as more fluid enters and forms a cavity, the blastocyst cavity. These outer cells flatten and are called trophoblast cells, since their role is limited to absorbing nutrition for the developing embryo. The inner, larger cells form the inner cell mass, or embryoblast, from which the embryo proper will actually develop; these remain attached to the trophoblast on only one side. The trophoblast cells in direct contact with the embryonal knob are specifically called the cells of Rauber. At this stage, the blastocyst shows a distinct polarity: the side bearing the inner cell mass is the embryonal end, and the opposite side is the abembryonal end. …