60 BLOOD AND BLOOD-VESSELS 



not clearly distinguishable. The peripheral layer of cells becomes the endo- 

 thelium of the vessel- wall, while the central mass is resolved into the primitive 

 nucleated blood-corpuscles. The islands are united together by cellular processes 

 which, becoming hollow, produce a continuous network of vessels. 



From the third week onwards until the liver is developed, it appears from the descriptions 

 of Graf v. Spee that the wall of the yolk-sac becomes converted into a tissue resembling liver- 

 tissue in its simplest form. Saccular dilatations of the entodermic lining of the vesicle are 

 produced, and from the walls of these dilatations solid masses of cells are budded off. Among 



FIG. 85. VASCULAB ABEA OF THE BABBIT OF ELEVEN DAYS. (v. Beneden and Julin.) 



The arteries are represented red, the veins blue ; the capillaries are not shown. The terminal sinus 

 is seen to be arterial. 



the cells are seen ' giant ' elements, derived possibly from the epithelial cells, and within these 

 are smaller cells closely resembling young nucleated blood -corpuscles. 



Once formed, the blood-vessels on the yolk-sac are in direct continuity with vessels which 

 develop in the connecting stalk, and through them with the vessels of the chorion. In this 

 xespect the conditions in the Primates again differ from those prevailing in the lower mammals. 

 Thus Selenka has shown that in Hylobates rafflesi the vessels on the under aspect of the yolk-sac 

 communicate with the vessels of the chorion by a pair of vessels surrounding the allantoic tube, 

 before there are any vessels in the embryo itself. This arrangement seems to be present in the 

 human embryo also, for a similar vascular loop was described by Eternod in his young embryo, 

 and named by him the sinus ensiforme. 



