Structure and development of 125 guinea pig placentae from the 14th day of pregnancy to full term were studied by light-microscope. At 14 days the placenta has the shape of a cone. It consists of a network of uncapillarized syncytial trophoblast, which is covered at the outside by a thin layer of cytotrophoblast. From this Duval's giant cell layer differentiates between 16 and 21 days. Until 23rd day its secretions form the Reichert's membrane. From 16 days onwards the transformation of uncapillarized syncytial trophoblast into labyrinth-tissue begins by an ingrowing of fetal mesenchyme cells. At the same time the largest part of the small newly formed syncytial nuclei degenerates. At 23 days the fetal mesenchyme is supplied with capillaries so that the vessels lie adjacent to the fetal, still unvascularized trophoblast or partially even penetrate deep into it. The proliferation of the capillaries illustrates the tendency of the labyrinth to expand, which lasts until full term. In the course of development the capillarized syncytium acquires the shape of a wheellike plate, which has short pegs towards the fetus and longer ones in basal direction. In the labyrinth a central zone with very wide blood spaces and capillaries as well as more basophil cytoplasma differentiates early. Together with the residual peripheral labyrinth and the still existing uncapillarized trophoblast 3 different regions can be distinguished by light-microscopy (lobule centre, lobule periphery, interlobium), which undergo typical changes in volume during further development. With injection-preparations the vascularisation of the placenta was studied. It was shown that maternal venous blood spaces and fetal arteries on the one hand and maternal arterial blood spaces and fetal veins on the other run together. In the lobule the maternal and fetal blood vessels form a threedimensional network of capillaries and bloodspaces in which a modified counter-current-flow of the blood occurs. © 1969 Springer-Verlag.
