370 



ectoderm also loses its cellular character and spaces separated by 

 protoplasmic bridges appear between the two layers. 



The enclosed maternal capillaries increase in size and in the 

 last day of gestation, maternal blood is often found to have passed 

 into the just mentioned spaces, some of which become enlarged to 

 form enormous sinuses in the syncytial protoplasm. 



The portion of the yolk-sac placenta under consideration is thus 

 constituted by a conjoint layer formed by the intimate fusion of 

 foetal and maternal tissues. The foetal part of the layer consisting 

 of a trophoblastic syncytium with large deeply staining nuclei and 

 derived from the ectoderm and entoderm of the bilaminar omphalo- 

 pleure, encloses the maternal portion viz. maternal capillaries and 

 isolated groups of degenerate uterine epithelial cells as well as 

 sinuses filled by maternal blood. 



Without doubt this portion of the yolk-sac placenta is functional 

 in the elaboration of nutriment derived from the maternal blood and 

 its transmission directly into the cavity of the yolk-sac. In this con- 

 nection it is worthy of note that the ectodermal nuclei of the syn- 

 cytium increase in size with the development of the layer and that 

 in the completed structure they are large, rich in chromatin, and 

 deeply staining. 



As regards the two portions of the yolk-sac placenta it may 

 even be that we have here a division of labour, that part of it 

 derived from the vascular omphalopleure serving perhaps mainly for 

 gaseous interchange, the second portion being solely concerned in 

 furnishing the necessary nutriment. That this second portion is of 

 very considerable functional importance is suggested by the compara- 

 tively poor development of the capillary system of the first part of 

 the placenta. 



In the region under consideration the openings of the uterine 

 glands are occluded by thin portions of the omphalopleural syncytium, 

 while in the first part of the placenta the vascular omphalopleure 

 similarly passes continuously over the openings. 



2) Over the lower polar area of the yolk-sac there is developed 

 no such intimate union with the uterine epithelium as has just been 

 described. The omphalopleure is here on the whole a thin attenuated 

 layer which does not closely follow the contour of the deeply folded 

 uterine surface, but is only attached to the ridges at intervals. The 

 uterine epithelium has nevertheless undergone a considerable amount 

 of prolifei'ation though maternal capillaries are in no wise specially 

 abundant in relation to it. This lower portion of the omphalopleure 



