104 



THE EARLY DEVELOPMENT OF MAMMALS. 



Clio 



completed, the essential relations are found to be as illustrated by the diagram 

 (Fig. 48, 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 modification in the 



structure of the chorion, which is of 

 the greatest morphological impor- 

 tance. This modification is the de- 

 velopment 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 neighborhood of the allantoic 

 diverticulum. Two of these vessels 

 are veins and two are arteries. They 

 are termed the umbilical vessels. The 

 veins at the embryonic end of the 

 body-stalk enter the somatopleure 

 of the embryo, through which they 

 make their way toward the heart. 

 The umbilical arteries, where they 

 join the embryo, are found to unite 

 and join the main aorta, so that they 

 maybe termed the terminal 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 

 diverticulum 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. 



Am. 



Emb. 



All. 



b.s. 



Clw. 



Fig. 48. — Diagrams illustrating the Relations 

 of the Allantois in Unguiculate Mammals. 



A, Before, Ii, after the formation of the amnion. All, 

 Entodermal allantois. Am, Amnion. b.s, Body- 

 stalk. Cho, Chorion. Ca, Extra-embryonic ccelom. 

 Emb, Anterior end of embryo. Yk, YolU-sac. 



