1905.] 



PLACENTA OF THE SPINY MOUSE. 



285 



The advancing edge of the trophoblast is sharply defined from 

 the matei-nal tissue (text-fig. 44, T). The troj^hoblast is composed 

 of a compact mass of cells with large nuclei and generally faii-ly 

 well-mai'ked 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-fio;. 45. 



A diagram of a section taken thvougli the centre of the placenta of Acnmys. The 

 maternal channels are not much exaggerated in size, but the foetal capillaries 

 are considerablj^ magnified. The trophoblast la_ver 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. T). Layer of cell-detritus probably 

 foetal as well as maternal. FC. Foetal capillary. H. Hypoblast of the parietal 

 wall of the yolk-sac. HH. Hj^poblast of the proximal wall of the yolk-sac, 

 very much folded. L. Lacunae in trophoblast containing maternal blood. 

 M. Muscle-coat of uterus. MA. Maternal artery. MES. Mesoblast of foetus. 

 MV. Maternal vein. S. Line along which the placenta had been detached 

 from the uterus. SP. Splanchnopleur layer of mesoblast. T. Trophoblast. 

 TS. Trophospongia. VV. Approximate portion of the main vitelline vessels. 

 Y. Cavity of the yolk-sac. YC. Blood-vessels of the j'olk-sac circulation. 



This trophoblastic tissue, which in the figure (text- fig. 45) is 

 shown as a thick black mass (T), is honeycombed by channels 

 containing maternal blood (L), which channels become moi-e 

 broken and more numerous nearer to the foetal 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 



