MOO aa| a PLACENTA OF THE SPINY MOUSE. 283 
and containing dead nuclei, which [ must regard as cell-detritus. 
This layer probably lay against, and was no doubt more or less 
attached to, the uterine wall (text-fig. 45, D). 
Text-fig. 45. 
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 about the 
channels containing maternal blood. 
F.BV. Feetal capillary. LE. Maternal leucocyte. MCH. Maternal blood in 
channels excavated in the trophoblast of the fetus. T. Trophoblast 
nuclei. > 480. 
From this point and passing over the edge of the placenta, and 
covering the free surface of the feetal side of the placenta, a flat- 
tened attenuated epithelium can be distinguished (text-fig. 45, H, 
p. 285). This becomes thicker and more 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 under 
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 ingrowth 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 
