1905.] PLACENTA OF THE SPINY MOUSE, 283 



and containing dead nuclei, which I must regard as cell-detritus. 

 This layei' probably lay against, and was no doubt more or less 

 attached to, the uterine wall (text-fig. 45, D). 



Text-fie-. 43. 



A section of a portion of the placenta of Acomys taken vertically near the centre of 

 the organ, where the foetal capillaries are forming a network round ahout the 

 channels containing maternal blood. 



F.BV. Foetal capillary. LE. Maternal leucocyte. MCH. Maternal blood iu 

 channels excavated in the trophoblast of the foetus. T. Tvophoblast 

 imclei. X 480. 



From this point and passing over the edge of the placenta, and 

 covering the free surface of the fcetal side of the placenta, a flat- 

 tened attenuated epithelium can be distinguished (text-fig. 45, H, 

 p. 285). This becomes thicker and moi-e cubical as it nears the 

 point at which the yolk-sac wall is connected with the placenta, 

 and here it passes into the decidedly cubical or columnar epithelium 

 of the yolk-sac. This layer continued in the other direction 

 would pass at some period into the distal wall of the yolk-sac, 

 though whether this distal wall exists at the period undei- 

 examination I cannot say. 



The rough surface of vascular attachment, so far as I can 

 judge from the general character of the cells, is composed entirely 

 of maternal tissue. This tissue is of that kind so frequently 

 found where trophoblastic ingi'owth is about to take place, and 

 had been named by Hubrecht trophospongia (text-fig. 44, p. 284). 



Text-fig. 45 is a diagrammatic representation of a section 

 passing through the centre of the placenta. The placenta, as 



