MAMMALIAN DEVELOPMENT 



819 



In mammals, the allantois spreads between the amnion and serosa 

 in about the same way, and becomes fused with the inner side of the 

 serosa to form the chorion. Many branched processes, chorionic villi, 

 extend from the outer or serosa surface of this membrane and come 

 to fit into corresponding pits in the internal uterine wall of the 

 mother. 



The yolk sac is another membrane which extends ventrally from 

 the mid-gut and out through the ventral side of the body wall just 

 anterior to the allantois. This structure is present in the fish and 

 amphibia as well as in the amniotes. It is large in the shark for 



Embryo 

 Ectoderm 



Ectoderm 

 Parietal 

 Mesoderm 



Splanchnic 

 Mesoderm 



Endoderm 



Yolk Sac 



Parietal 

 Mesoderm 



Fig. 424. — ^Diagram showing development of tlie embryonic membranes of a 

 vertebrate. The amnion and serosa (chorion) each consists of a fusion of 

 ectoderm and somatic mesoderm ; while the yolli sac and allantois each consist 

 of a fusion of endoderm and splanchnic mesoderm. The serosa is composed of 

 the two outer layers of the diagram. Notice that the allantois is a double-walled 

 sac from the ventral stalk, and it extends well around the embryo. 



Amnion 



■Amniotic Cai^ity 



■Allantois 

 'Parietal 



Mesoderm 



Splanchnic 

 Mesoderm 

 Yolk Sralk 



Endoderm 



Vitelline 

 Membrane 



example. In reptiles and birds it carries the large mass of yolk used 

 for nutriment by the embryo. In mammals it is much reduced be- 

 cause the embryo soon develops a means of nourishment through the 

 placenta. 



Placenta. — This membranous sac in which the embryo and later 

 the fetus (designation after distinct body form is apparent) is 

 formed from two sources, one embryonic and the other maternal. 

 As suggested in the paragraph above, the chorion of the embryo and 

 the mucous membrane (internal lining) of the uterus unite (actually 



