The villous stems of the human placenta.

Dissection and graphic reconstruction of villous stems of human placentas has shown that the allegedly typical pattern, namely, one single villous stem and one maternal arterial ostium supplying each lobule, is not the usual state. More often, portions of one villous stem supply several lobules, and several stems take part in the formation of one lobule. This, together with the previously demonstrated location of arterial ostia at the interlobular areas, leads to the concept that arterial ostia, usually in close association with large anchoring villi, form a network at the base of the placenta which determines the distribution of the lobules.