Grosser’s classification of placentas by the number and form of the layers intervening between the fetal and maternal circulations is generally considered the most useful and instructive yet devised (Amoroso, 1952; Steven, 1975). However, it needs to be recognized as a purely anatomical categorization with no necessary corresponding physiological efficiencies nor evolutionary predictions. It is therefore as important to incorporate new anatomical evidence to update the scheme as it is to consider the possible functional relevance of the differences. Previous suggested extensions to the scheme such as hemoendothelial (Owers, 1960) or endothehoendothelial (Mossman, 1937) placentation have been shown by electron microscopic investigation to be invalid because a continuous fetal endothelium and chorion always persist albeit very thinly in places. However, recent electron microscope and other work has demonstrated that the syndesmochorial category needs re-assessment. The ruminant placenta was defined as syndesmochorial because Grosser considered that the uterine epithelium disappeared and the fetal trophectoderm was apposed directly to the maternal connective tissue (Borlands Medical Dictionary, syndesmo = related to connective tissue). Subsequent workers demonstrated that the uterine epithelium persisted (sometimes in a syncytial form) and therefore reclassified the ruminant placenta as epitheliochorial (Ludwig, 1962; Steven, 1975, 1983; Ramsey, 1982). In the last decade evidence has accumulated to show that a unique feature of the ruminant placenta is the population of fetal chorionic binucleate cells (BNC) which migrate throughout pregnancy through the chorionic tight junction to fuse with uterine epithelial cells or their derivatives [Figures l(a) and (b)]. The horse placenta produces migratory binucleate cells (Allen, Hamilton and Moor, 1973) but these are transient and never fuse with maternal cells. Ruminant BNC are directly involved in the modification of the uterine epithelium, beginning at implantation and continuing until term (Wooding, 1982, 1983; Wooding et al, 1986). The uterine epithelium persists but is modified to a variable degree, depending on species, into a hybrid fetomaternal syncytium formed by the migration and fusion of the fetal binucleate cells with those of the uterine epithelium. The mature ruminant placenta is neither entirely syndesmochorial with no uterine epithelium, nor epitheliochorial with two apposed cell layers whose only anatomical interaction is interdigitated microvilli. Thus in sheep and goats all the placentoma1 membrane, more than 95 per cent of the total (placentomal plus interplacentomal) in the second half of pregnancy, form a fetomaternal syncytium at the fetomatemal interface IFigure l(b); Wooding, 1984; Wango et al, 1990a,b]. In cow and deer, although at