92 



THE EARLY DEVELOPMENT OF MAMMALS. 



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. 41, hi. 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 the cells have become free the amount of protoplasm in each cell 

 increases. The cells multiply rapidly by mitotic division. According to the 

 prevalent hypothesis, all of the colored blood-corpuscles are descendants of these 

 cells derived from the blood-islands. 



The angioblast continues growing by the development of buds from the 



Fig. 41. — Section of the Area vasculosa of a Chick Embryo of the Second Day. 

 Som, Somatopleure. Spt, Splanchnopleure. Ec, Ectoderm. En, Entoderm. bl.is, Blood-islands. V, V, 



Blood-vessels. X 227 diams. 



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 become continuous, and thus the vascular network is extended. 



The Development in Mammals. — The origin of the blood-vessels in mam- 

 mals 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 mammals that the entire yolk-sac, probably owing to its small 

 size, becomes, very early indeed, vascularized throughout. 



The Growth of the Vessels into the Embryo. — The entrance of the vessels into 



