150 T. THOMSON FLYNN 



cells are often rounded, but in other cases possess an irregular 

 profile suggesting the presence of pseudopodial processes. 



No doubt the vacuolation of the ectodermal cells of this 

 region of the blastocyst is to be associated with an active 

 absorption on the part of these cells of carbohydrate (? glycogen) 

 in process of being transferred to the embryo per medium of 

 the yolk-sac. The cells closely resemble the glycogenic cells 

 figured by authors for certain other mammals (e. g. Jenkinson, 

 1902, I. vi). 



The entoderm of this region consists of a layer of cells 

 somewhat darkly staining with haematoxylin. These are 

 sometimes rounded, sometimes flattened. I have found the 

 entoderm cells occasionally slightly vacuolated in the manner 

 described by Hill for his species. 



It is a well-known fact that the vacuolation of cells con- 

 cerned in embryotroi)hic processes varies greatly at different 

 stages. Hill found that the entoderm cells of the yolk-sac, but 

 slightly vacuolated in early stages, became greatly so in his 

 12-5 mm. specimen. This he regarded as a forerunner of 

 degeneration, but I rather think it is to be associated with the 

 active absorption and internal transmission by the yolk-sac 

 wall of substances (probably carbohydrate) secreted by the 

 mother and destined for the nutrition of the foetus. 



The Yolk-sac Placenta. — As stated above, this is 

 brought about by the intimate apposition of the vascular 

 omphalopleure to the extra-placental portion of the uterine 

 wall. Fig. 23 shows the details of this circulation and fig. 24 

 is a somewhat diagrammatic representation of the area with 

 regard to the blastocyst wall and the em])ryo. 



In fig. 23 most of the bilaminar omphalopleure has been 

 removed and the remainder of the blastocyst wall has been 

 reflected over the embryo. The vascular area is therefore seen 

 from its inferior aspect. The anterior end of the embryo is 

 contained in the proamnion {proa.), here large and persistent. 



The vitelline artery (vit.n.) is a very large and thick trunk 

 which, after leaving the yolk-stalk, keeps at first a little to the 

 left side of the body and passes over the surface of the tail to 



