88 THE EARLY DEVELOPMENT OF MAMMALS. 



ation is going on, and it can be easily seen that it is most advanced in those cells 

 which are nearest the embryo and least advanced in those cells which are nearest 

 to the germinal wall. In mammals the area pellucida is well marked and re- 

 sembles that of birds. The area opaca has well-defined cylinder cells (Fig. 36) 

 which have rounded ends, but are much smaller than in birds and contain very 

 little yolk material. Cells of this character extend over also what we should call 

 the area vitellina, which does not present the special features which it has in 

 birds, for the reason that the yolk in mammals is so small in amount. Later on 

 the cells pass through degenerative changes, which need to be more exactly 

 studied. In man the degenerative 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 



FIG. 37. -SECTION OF THE YOLK-SAC OF A YOUNG FlG . 3 8._HuMAN EMBRYO, 2.15 MM. LONG. 



HUMAN EMBRYO. (AJter W. His.} 



Ent, Entoderm. mts, Mesoderm. v, Blood- 

 vessels. (After Keibel.} 



makes its first appearance in the region of the area opaca, where it forms a net- 

 work 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 surrounded by a region of extra-em- 

 bryonic blood-vessels the definitive area vasculosa. Now, it will be remem- 

 bered 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 



