112 



THE EARLY DEVELOPMENT OF MAMMALS. 



Cce. 



Cho. 



Yk. 



the extra-embryonic somatopleure or primitive chorion, Cho. Later, when the for- 

 mation of the amnion is completed, the essential relations are found to be as 

 illustrated by the diagram (Fig. 64, B). The amnion arises from the distal end 

 of the body-stalk, but the body-stalk retains its connection with the chorion. When 

 the allantois becomes free, the connection with the chorion is entirely lost. The 

 maintenance of that primitive connection in the unguiculates is to be regarded as a 

 new modification of the relations of the embryonic appendages, evolved only in 



the higher animals. The maintenance of 

 that connection makes possible the modi- 

 fication in the structure of the chorion, 

 which is of the greatest morphological 

 importance. This modification is the 

 development of the blood-vessels in the 

 chorion. The anlages of these blood- 

 vessels are outgrowths of the embryonic 

 angioblast. They appear so as to form 

 four vessels which grow through the 

 length of the body-stalk in the neighbor- 

 hood of the allantoic diverticulum. Two 

 Am. of these vessels are veins and two are 

 Emb arteries. They are termed the umbilical 

 vessels. The umbilical veins at the 

 Cce. embryonic end of the body-stalk enter 

 the somatopleure of the embryo (Fig. 

 Yk. 186, V. U.S. , V. U.D), through which 

 they make their way toward the heart 

 (Fig. 93, Alv). The umbilical arteries 

 enter the body of the embryo, pass 

 caudad alongside the allantois (Fig. 210, 



A, Before, B, after the formation of the amnion. All, A.um), Curve past the cloaca onto the 



Entodermal allantois. Am, Amnion. b.s, Body- dorsal side of the body (Fig. 169, A Mm), 

 stalk. Cho, Chorion. Cos, Extra-embryonic 



coelom. Emb, Anterior end of embryo. Yk, and J om the Caudal Cnd f the a Fta > 



Yolk-sac. so that they may be termed the termi- 



nal branches of the embryonic aorta. 



In early stages they are the largest branches which the aorta has. At the distal 

 end of the body-stalk the four vessels enter the mesoderm of the chorion, there 

 branch abundantly, and produce a rich network of blood-vessels throughout the 

 entire membrane. The unguiculate mammals, therefore, are characterized by this 

 special feature, the possession of the body-stalk which contains the allantoic diver- 

 ticulum and gives access for the blood-vessels, and therefore also, of course, for the 

 blood, to the chorion, which thus becomes vascular. In all other amniota the 

 chorion is without blood-vessels. 



Cho. 



FIG. 64. DIAGRAMS ILLUSTRATING THE RELATIONS OF 

 THE ALLANTOIS IN UNGUICULATE MAMMALS. 



