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Show 286 MR. R. ASSHETON ON THE FCETUS AND [June 6, blood makes up nearly half the thickness of the placenta, and contains no foetal mesoblast or blood. The half of the placenta towards the foetal surface is made up of trophoblast (much attenuated) forming channels filled with maternal blood, which take a more or less sinuous course, and a network of fine foetal capillaries, with also the larger vessels and larger main maternal channels. This is shown diagrammatically in text-fig. 45, FC, p. 285). Text-fig. 43 (p. 283) is a drawing of an actual section of a piece of this region near the maternal surface. The great bulk is made up of the channels (MCH) excavated in the trophoblast containing maternal blood. There are many leucocytes (LE). The walls of these channels are thin, though the large trophoblastic nuclei (T) are very conspicuous. The foetal capillaries are seen at F.BY. Nearer to the foetal surface the maternal channels become finer and the foetal capillaries perhaps rather more numerous. At places where the main foetal arteries penetrate the tissues of the placenta, a considerable quantity of ffetal mesoblast tissue accompanies them. There are a few spherical masses of tissue within this region, which are not vascular, nor do they seem to be trophoblastic. They resemble in some respects Duval's " llots vesiculeux," which, according to him, are pieces of the maternal sub-mucosa which have become enveloped by the advancing trophoblast layer. The main features of the vascular systems are fairly easily determinable. In this specimen the whole of the maternal arterial blood-supply arises from a single artery in the centre (MA), which opens into the large afferent channel which lies partly in the trophospongial tissue and partly in the trophoblast. This, like the other main channels, is lined by a flattened epithelium-like layer, which is probably a pseudo-epithelium of trophoblastic origin where the wall is trophoblast, and trophospongial origin where the wall is trophospongia. Duval has described the growth inwards along the maternal vessels of trophoblast cells to form a pseudo-epithelium. This is denied by Jenkinson*, who derives the pseudo-epithelium from the simple flattening of the adjacent cells. This is not a question which can be decided by reference to a single stage; but I may say that there is nothing in this specimen which supports in any way Duval's account in the mouse. The afferent channel divides into two main branches, which diveige and then penetrate straight to the foetal surface of the placenta. Here they break up into channels, which take a rather more sinuous course back again to the middle of the thickness of the placenta, where they collect into a number of efferent channels lying near the surface of the trophoblast and ultimately into two * Jenkinson, J. W., " Observations on the Histology and Physiology of the Placenta of the Mouse," Tijdschr. d. Ned. Dierk. Vereen. Dl. vii. 1902. |