286 ME. R. ASSHETON ON THE FOETUS AND [Juiie 6, 



blood makes up nearly half the thickness of the placenta, and 

 contains no fcetal 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, FO, p. 285). 



Text-fig. 43 (p. 283) is a drawing of an actual section of a piece 

 of this region near the maternal siu'face. 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.BV. 



Nearer to the fcetal surface the maternal channels become 

 finer and the fcetal capillaries perhaps rather more numerous. 

 At places where the main foetal arteries penetrate the tissues of 

 the placenta, a considerable quantity of foetal 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 " ilots 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), whicli 

 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 tropho- 

 spongial origin where the wall is trophospongia. 



Duval has described the growth inwards along the maternal 

 vessels of troj^hoblast 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 movise. 



The afferent channel divides into two main branches, which 

 divei'ge and then penetrate sti-aight to the foetal surface of the 

 placenta. Here they break up into channels, which take a rather 

 more sinvious 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., " Obsei-vations on the Histology and Plij'siology of the Placenta 

 of the Mouse," Tijdschr. d. Ned. Dierk. Vereen. 1)1. vii. 1902. 



