1905. ] PLACENTA OF THE SPINY MOUSE. 285 
The advancing edge of the trophoblast is sharply defined from 
the maternal tissue (text-fig. 44, T). The trophoblast is composed 
of a compact mass of cells with large nuclei and generally fairly 
well-marked cell-boundaries. It has, in fact, the character more 
of a cytotrophoblast than a plasmodi-trophoblast (text-fig. 44). 
The nuclei tend to arrange themselves in pairs. 
Text-fig. 45. 
MA 
A diagram of a section taken through the centre of the placenta of Acomys. The 
maternal channels are not much exaggerated in size, but the fcetal capillaries 
are considerably magnified. The trophoblast laver in the region of the foetal 
capillaries is not nearly so much attenuated as it should be. 
The foetal vessels are dotted, the channels containing maternal blood are white. 
The deep black is trophoblast. 
AA. Allantoic artery. AV. Allantoic vein. D. Layer of cell-detritus probably 
foetal as well as maternal. FC. Fetal capillary. H. Hypoblast of the parietal 
wall of the yolk-sac. HH. Hypoblast of the proximal wall of the yolk-sac, 
very much folded. L. Lacune in trophoblast containing maternal blood. 
M. Muscle-coat of uterus. MA. Maternal artery. MES. Mesoblast of fcetus. 
MV. Maternal vem. S. Line along which the placenta had been detached 
from the uterus. SP. Splanchnopleur layer of mesoblast. ‘IT. Trophoblast. 
TS. Trophospongia. WV. Approximate portion of the main vitelline vessels. 
Y. Cavity of the yolk-sac. YC. Blood-vessels of the yolk-sac circulation. 
This trophoblastic tissue, which in the figure (text-fig. 45) is 
shown as a thick black mass (T), 1s honeycombed by channels 
containing maternal blood (LL), which channels become more 
broken and more numerous nearer to the fetal surface, 
and the trophoblast consequently more attenuated. I think 
there are possibly other cavities in the trophoblast-cells which 
are not blood-spaces. 
This mass of tissue formed entirely of trophoblast and maternal 
