Although there is a fascinating variety of morphological relationships at different stages of implantation in various species, some common patterns can be discerned. The position within the uterus where blastocysts are found after transport is the end result of the activity of the uterine musculature, the endometrical conformation, and the size and shape of the blastocyst. Blastocysts of some species are situated in the approximate position where they will implant for an appreciable period of time. Apposition, brought about by blastocyst swelling or uterine lumincal closure, is the necessary prerequisite for the initiation of adhesion. Adhesion per se is progressive and involves the surface coats of the trophoblast and uterine luminal epithelial cells. It maybe thought to include such events as the development of adhesive factor(s) on the trophoblast and the local deletion of some of the uterine coat constituents. When adhesion is followed by invasion of the uterine mucosa, the trophoblast adheres closely to the uterine epithelial cells, sharing junctional complexes with healthy cells, and engulfing displaced or surrounded cells.