By routine EM preparative techniques, the tissues which, collectively, separate maternal and fetal bloods in the fully formed chorioallantoic placenta of the rat have been shown to consist of three chorionic layers, or trophoblast, and a layer of allantoic capillary endothelium [Fig. 1]. Relationships between these layers are best demonstrated by special techniques, viz., cacodylate-buffered aldehyde fixation, collidine-buffered osmium tetroxide postfixation, and en bloc staining with uranyl acetate. By using this method on placentas at term, the cells of the outermost chorionic layer (Trophoblast 1) appear to be attached to each other by means of maculae adherentes which sometimes occur in clusters [Fig. 2].
