92 



THE EARLY DEVELOPMENT OF MAMMALS. 



have at first a rounded or branching form. In the inner part of the pellucida they are 

 small and stand alone. Toward the periphery they are larger, more closely set, 

 and more united with one another. Their development is greater around the caudal 

 end of the embryo. 



In the next stage the vascular anlages become hollow, and then may be 

 called true blood-vessels. When they acquire a lumen, the blood-islands are found 

 to remain attached usually to the upper side of the vessel like a thickening of its 

 wall (Fig. 51, bl.is). Very soon after the vessels have become hollow the cells of 

 the blood-islands break apart and lie free in the cavity of the vessel, thus forming 

 the first blood-corpuscles. They are characterized by having a rounded nucleus 

 with a very distinct nucleolus, and a minimal covering of protoplasm only. After 



som 



FIG. 51. SECTION OF THE AREA VASCULOSA OF A CHICK EMBRYO OF THE SECOND DAY. 



Som, Somatopleure. Spl, Splanchnopleure. EC, Ectoderm. En, Entoderm. bl.is, Blood-isands. V, V, Blood- 

 vessels. X 227 diams. 



the cells have become free the amount of protoplasm in each cell increases. The 

 cells multiply rapidly by mitotic division. It is believed that all the blood- 

 corpuscles, both red and white, are descendants of these cells derived from the 

 blood-islands. 



The angioblast continues growing by the development of buds from the vessels 

 already formed. These buds are rounded or pointed, forming, as it were, spurs. 

 They often end by meeting one another and uniting. They are usually hollow from 

 the first, and after they meet one another or an adjacent vessel, the cavities be- 

 come continuous, and thus the vascular network is extended. 



The Development in Mammals. The origin of the blood-vessels in mammals is 

 not adequately known. The solid primary anlages appear in the extra-embryonic 

 area vasculosa and extend later into the embryo. They present well-marked blood- 

 islands, which make their first appearance in rabbit embryos of the eighth day, 

 just before the appearance of the first primitive segments. It is characteristic of 

 most mammals that the entire yolk-sac, probably owing to its small size, becomes, 

 very early indeed, vascularized throughout. 



