66 



THE EARLY DEVELOPMENT OF MAMMALS. 



erative change in the cells of the yolk-sac takes place very early. The mesoderm 

 of the yolk-sac is at first a thin layer. Very early there appears an angioblast, or 

 the anlage of the first blood-vessels and blood. In all cases in which the process 

 has been accurately followed the angioblast makes its first appearance in the region 

 of the area opaca, where it forms a network of primitive blood-vessels close 

 against the surface of the yolk. The region occupied by these blood-vessels is 

 called the area vasculosa. Its boundary in the direction away from the embryo is 

 everywhere well defined. Gradually the development of blood-vessels progresses 

 from the region of the area opaca into the region of the area pellucida and extends 

 into the body of the embryo. We even have the embryo almost completely sur- 



raes 



FIG. 32. SECTION OF THE YOLK-SAC OF A YOUNG 



HUMAN EMBRYO. 

 En, Entoderm. mes, Mesoderm. v, Blood-vessels. 



(After Keibel.) 



FIG. 33. HUMAN EMBRYO, 2.15 MM. LONG. 

 (After W. His.) 



rounded by a region of extra-embryonic blood-vessels the definitive area vasculosa. 

 Now, it will be remembered that the area opaca is the territory in which the 

 entodermal cells are actively assimilating the yolk, and we must believe that the 

 blood-vessels which 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 per- 

 manent 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. 32).. 



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

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



