In an effort to define further the factors that can influence trophoblastic growth, development of the discoidal chorioallantoic placenta was examined histologically in laboratory-bred black mastiff bats, Molossus ater. This placenta normally forms only at the cranial end of the right uterine horn. Its positioning was found to be related to the development of an unusual vascular tuft in the endometrium at this site following ovulation. When a decidual reaction occurred, the endothelial cells of the tuft vessels hypertrophied and their basal laminae became more prominent in sections stained for glycoproteins. Cytotrophoblast then proliferated preferentially around the tuft in the right horn and its vessels became surrounded by trophoblastic cuffs. A functional placenta was formed when trophoblastic tubules grew out from these cuffs, became interconnected, developed lumina, and began to carry maternal blood. Maternal endothelial cells generally persisted only in portions of the larger vascular tubules. These observations suggest that the endothelial cells of the tuft vessels may be secreting factors that influence early trophoblastic growth and are at least partially incorporated into the basal laminae of the endothelial cells. Evidence is reviewed which raises the possibility that endometrial endothelial cells might have a similar morphogenetic role in many other mammals with invasive trophoblast. Mitotic activity in the trophoblastic tubules of M. ater ceased when the tubules became patent and began to carry blood. Further growth was then accomplished by hypertrophy of the existing cytotrophoblast cells. Electron microscopic examination of near-term placentas confirmed that the interhemal barrier was hemomonochorial and lacked a continuous layer of syncytiotrophoblast.