MAMMALIA. 251 



is formed the usual subzonal membrane, which soon becomes separated by 

 a considerable space from the yolk-sack (fig. 153). The yolk-sack is, how- 



FIG. 153. EMBRYO AND FOETAL MEMBRANES OF A YOUNG EMBRYO ROE-DEER. 



(After Bischoff.) 



yk. yolk-sack; all. allantois just sprouting as a bilobed sack. 



ever, continued into two elongated processes (yk), which pass to the two 

 extremities of the subzonal membrane. It is supplied with the normal 

 blood-vessels. As soon as the allantois appears (fig. 153 all], it grows out 

 into a right and a left process, which rapidly fill the whole free space within 

 the subzonal membrane and in many cases, e.g. the Pig (Von Baer), break 

 through the ends of the membrane, from which they project as the diver- 

 ticula allantoidis. The cavity of the allantois remains large, but the 

 lining of hypoblast becomes separated from the mesoblast, owing to the 

 more rapid growth of the latter. The mesoblast of the allantois applies 

 itself externally to the subzonal membrane to form the chorion 1 , and in- 

 ternally to the amnion, the cavity of which remains very small. The 

 chorionic portion of the allantoic mesoblast is very vascular, and that 

 applied to the amnion also becomes vascular in the later developmental 

 periods. 



The horns of the yolk-sack gradually atrophy, and the whole yolk- 

 sack disappears some time before birth. 



Where two or more embryos are present in the uterus, the chorions of 

 the several embryos may unite where they are in contact. 



From the chorion there grow out numerous vascular villi, which fit into 

 corresponding pits in the uterine walls. According to the distribution of 

 these villi, the allantois is either diffused or polycotyledonary. 



The pig presents the simplest type of diffused placenta. The villi of 



1 According to Bischoff the subzonal membrane atrophies, leaving the allantoic 

 mesoblast to constitute the whole chorion. 



