11.5 Human cleavage and compaction. There is a slight growth advantage to XY blastomeres that may have had profound effects for in vitro fertility operations. The mechanisms of implantation. The molecular mechanisms by which the blastocyst adheres to and enters into the uterine wall constitute a fascinating story of cell adhesion and reciprocal interactions between two organisms, the mother and the embryo. 11.7 Placental functions .
Placentas are nutritional, endocrine, and immunological organs.
They provide hormones that enable the uterus to retain the pregnancy and also accelerate mammary gland development. Placentas also block the potential immune response of the mother against the developing fetus. Recent studies suggest that the placenta uses several mechanisms to block the mother's immune response. 11.8 Non-identical monozygotic twins. Although monozygotic twins have the same genome, random developmental factors or the uterine environment may give them dramatically different phenotypes.