ABSTRACT
Cleavage starts about 24 hours after fertilization. The four-cell stage is reached 40 hours after fertilization; the 12<ell stage within 3 days; and the morula stage (16 to 32 cells) is seen on day 4. The cells of the morula segregate into trophoblasts (outer cell mass) and embryoblasts (inner cell mass). The cells (blastomere) at this stage are not determined cells. Their position in the morula determines their ultimate location. The outer larger cells with microvilli divide early to form the trophoblast. With the accumulation of fluid, the blastocyst is formed. The cells of the blastocyst are determined cells, and they differentiate into trophoblast and embryoblast. The embryoblast continues for a short time to add cells to the expanding trophoblast. During the first seven days, the cells undergo about eight divisions. The first week ends with onset of implantation.
