On the dorsal side, cortical cytoplasm rotates upward in relation to the underlying yolky inner cytoplasm that remains in place. The sperm entry point would be at the upper left. Most of the pigment granules are in the cortical cytoplasm of the animal pole, so the pigment boundary rotates upward on the dorsal side, revealing the more sparsely pigmented inner cytoplasm and thus delineating the gray crescent. J Implantation of Human Embryo . This animated movie shows the implantation into the uterine lining of a human embryo, as seen in section.
Implantation begins at the end of the first week, when the embryo attaches to the uterine wall.
The conceptus is completely beneath the uterine surface by the end of the second week. The outermost layer of the embryo, the syncytiotrophoblast, produces the enzymes to dissolve the maternal tissue, and the amount of syncytiotroblast increases dramatically during the second week. Nuclei remain unstained and thus appear as dark circles. The blastomeres undergoing meroblastic cleavage in the lower portion of the image are connected by cytoplasmic bridges to the uncleaved yolky part of the egg. During every prophase there is a major onset of motility of the yolk platelets.