FCETAL MEMBRANES. 571 



inner surface of the albumen sac, evidently for the purpose of increasing its 

 absorptive surface, this albumen sac has been compared by some to a placenta. 



3. It blends with the primitive chorion to form the true chorion and being 

 extremely vascular and lying just beneath the porous shell, it serves as the most 

 important organ of fcetal respiration. 



The allantois in the chick is an extremely vascular organ, the network of 

 small vessels in the wall being composed of radicals of the allantoic or umbilical 

 vessels of the embryo. Soon after the allantois begins to develop, two 

 branches the umbilical arteries are given off from the aorta near its caudal 

 end. These pass ventrally through the body wall of the embryo and thence 

 out via the umbilicus to break up into extensive networks of capillaries in the 

 mesodermal layer of the allantois. The capillaries converge to form the um- 

 bilical veins which pass into the embryo via the umbilicus and thence cephalad 

 to the heart. 



During the incubation period of the chick there are two extraembryonic sets 

 of blood vessels. One set, the vitelline (omphalomesenteric) vessels (p. 187), 

 is concerned with carrying the yolk materials to the growing embryo. The 

 other set, the umbilical (allantoic) vessels, is chiefly concerned with respiration 

 and carrying waste products to the allantois, but is' probably in part concerned 

 with conveying the albumen to the embryo. When the chick is hatched, and the 

 fcetal membranes are of no further use and disappear, the extraembryonic por- 

 tions of the blood vessels also disappear. The intraembryonic portions persist, 

 in part, as certain vessels in the adult organism. 



The Chorion or Serosa. This membrane is but little developed in the 

 chick as compared with Mammals, especially the Placentalia. Its mode of 

 origin as the outer leaves of the amniotic folds, cut off from the amnion by 

 dorso-medial extension of the mesoderm and body cavity, has been described 

 (P- S^S) It consists, as there shown, of extraembryonic ectoderm and parietal 

 mesoderm (Fig. 483) . As first formed it is confined to the immediate region 

 of the embryo and of the amnion to which it is later loosely attached. It soon 

 extends ventrally around the yolk where it forms what is sometimes designated 

 the skin layer of the yolk sac. The relation of the outer layers of the allantois 

 to the chorion has been described on page 570, and is illustrated in Fig. 486. 



FCETAL MEMBRANES IN MAMMALS. 



The development of the fcetal membranes in Mammals presents no such 

 uniformity as is found in Birds and Reptiles where it was possible to describe 

 their formation in the chick as typical for the two classes. In the different 

 Mammals much variation occurs, not only in the first appearance of the mem- 

 branes but also in their further development and ultimate structure. 



In some forms (rabbit, for example) the amnion develops in a manner very 



