Considering the placenta from the functional point of view as an exchange organ be- tween mother and foetus, it is noteworthy that placentas of all types have a common structural principle in that all placentas have structures enlarging the surface area avail- able for exchange, as is also the case with other organs with similar functions, such as the lungs, kidneys and intestine. It may therefore be assumed that a quantitative rela- tionship must exist between the structure of the placenta and its function. Accurate information on this relationship could contribute to better understanding of the pla- centa and of its function. The study of this relationship requires quantitative physio- logical and morphological data. Detailed data on quantitative morphology (morphometry) are available mainly for the human placenta. Even these data, however, are rather sparse as far as the early sta- ges of development are concerned, because most-studies relate to full term placentas (see Aherne, 1975; Bender et al. , 1974; Laga et aI. , 1973; Geissler et aI. , 1972; Baur, 1972; Cattoor, 1967; Aherne and Dunnill, 1966). We know of only a few morphomet- ric studies concerned with placentas of other animal species. These studies relate to in- vestigations on the development of the placenta of sheep (Stegeman, 1974) and of cat- tle (Baur, 1972), and measurements carried out on full term placentas of the guinea- pig (Millier et aI. , 1967) and on full term placentas of various other species (Baur, 1973).