IN those types of placentae in which the trophoblast is non-invasive (epitheliochorial) or only very weakly invasive (syndesmochorial) the trophoblast is cellular. However, in the endotheliochorial and haemochorial types of placentae the trophoblast is progressively more invasive and in those species in which it has so far been studied the trophoblast, during its invasive period, is syncytial in structure1. The purpose of this communication is to direct attention to the fact that the mouse trophoblast, probably the most invasive trophoblast, is cellular. It has been known from earlier work2,3 that the rat trophoblast in the definitive placenta is cellular, but one cannot assume from this that it is cellular during its invasive state, for it has been recently shown4,5 that in the rabbit the trophoblast in the definitive placenta is cellular whereas the implanting trophoblast is syncytial.