THE YOLK-SAC. 



89 



are thus early developed in close contact with the cells of this area are destined 

 to take up food material digested by the entodermal cells and carry it to the 

 embryo. Hence we interpret the early development of the extra-embryonic 

 vessels as due to physiological necessities. 



The mesoderm at first forms a very thin layer over the angioblast. It next 

 thickens by the multiplication of its cells, and we can then distinguish in it both 

 the outer mesothelium and the inner mesenchyma. The mesothelium is the 

 permanent external cover of the yolk-sac. The mesenchyma grows in between 

 the primitive blood-vessels, and finally penetrates, at least in part, between the 

 blood-vessels, and the entoderm of the yolk-sac, a condition which is reached 

 very early in the human embryo (Fig. 37). 



The human yolk=sac is characterized by its small size and by the precocious 



^•^^^^ 





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Fig. 39. — Human Embryo of 2.6 mm. — [After W. His.) 



expansion of the area vasculosa, so that in the very earliest stage known to us by 

 observation blood-vessels are found over the entire sac. At the beginning of 

 the third week the diameter of the yolk-sac is about equal to the length of the 

 embryo (Fig. 65). By the end of the third week the sac has become distinctly 

 pear-shaped, its narrower pointed end being that by which it is connected with 

 the intestinal canal of the embryo (Figs. 38, 39). The sac continues growing, 

 up to the end of the fourth week, after which it enlarges very slightly, if at all. 

 Its diameter is only from 7 to 1 1 mm. It is then a pear-shaped vesicle attached 

 by a long stalk to the intestine, the stalk having been formed by the lengthen- 



