90 



THE EAR'LY DEVELOPMENT OF MAMMALS. 



ing of the neck of the yolk-sac (Fig. 40), The cavity of the stalk early becomes 

 obliterated and the entoderm in the stalk disappears altogether. 



The Origin of the Blood=vessels and Blood. 



As stated above (pages 88 and 89), the first blood-vessels appear in the 

 circumscribed region in the mesoderm of the yolk-sac and lie close against the 

 entodermal cells of the area opaca. .The region which they occupy is termed the 

 area vasculosa. From the area vasculosa the development of blood-vessels ex- 

 tends, as stated, across the area pellucida into the embryo.* During these early 



Fig. 40.— Human Embryo ov 9.8 mm. Probable Age Thirty Days. X 5 diams. 



stages the only blood-vessels are in the splanchnopleure. After their formation 

 has extended into the body of the embryo, it spreads into the somatopleure also, 

 which, therefore, acquires its blood-vessels at a later stage. It should be noted, 

 however, that the development of the blood-vessels begins before the coelom has 

 been developed over the area vasculosa. While they are forming, the coelom 

 expands ; and after it has appeared, the primitive blood-vessels are found always 

 exclusively in the splanchnic mesoderm. 



Definition. — The essential part of a blood-vessel is its endothelial wall. In 

 early stages all the blood-vessels consist only of endothelium. Arteries and 



* It has been recorded that in lizards the vascular aniages appear first in the area pellucida. 



