78 



COMPARATIVE PHYSIOLOGY. 



THE FCETAL (EMBRTONIC) MEMBRANES OF 

 MAMMALS. 



The differences between the development of the egg mem- 

 branes of mammals and bu-ds are chiefly such as result from 



the absence in the 

 ed. sh. former of an egg-shell 



and its membranes, and 

 of yelk and albumen. 

 The mammalian ovnm 

 is inclosed by a zona 

 radiata (zona pellud- 

 da) surrounded by an- 

 other very delicate cov- 

 ering (vitelline mem- 

 brane). 



The growth of the 

 blastodermic vesicle 

 (yelk-sac) is rapid, and, 

 being filled with fluid, 

 the zona is thinned and 

 soon disappears. 



The germinal area 

 alone is made up of 

 three layers of cells 

 (Fig. 100), the rest of ' 

 the upper part of the 

 oosperm being - lined 

 with epiblast and hyp- 



FiG. 79. —Diagrammatic longitudinal section 

 oosperm of rabbit at an advanced stage of preg- 

 nancy (KOUilier, after Bischoff). a, amnion; ai, 

 allantois with its blood-vesBcls; e, embryo; de, 

 yelk-sac ; ed, ed', ed"^ hypoblastic epithelium of 

 the yelk-sac and its stalk (umbilical vesicle and 

 cord); fd, vascular mesoblastic membrane of the 

 umbilical cord and vesicle ; pi, placental villi 

 formed by the allantois and subzonal membrane; 

 »•, space filled with fluid between the amnion, 

 the allantois, and the yelk-sac ; sU sinus termi- 

 nalis (marginal vitelline blood-vessel) ; w, urach- 

 us, or stalk of the allantois. 



oblast, while the low- 

 er zone of the yelk-sac consists of epiblast only. 



Simple, non-vascular villi, serving to attach the embryo 

 to the uterine walls, usually project from the epiblast of the 

 subzonal membrane. In the rabbit they do not occur every- 

 where, but only in that region of the epiblast beneath which 

 the mesoblast does not extend, with the exception of a patch 

 which soon appears and demarkates the site of the future pla- 

 centa. The amnion and allantois are formed in much the same 

 way as has been described for the chick. 



At about the same period as these events are transpiring the 

 vascular yelk-sac has become smaller, and the allantois with 

 its abundant supply of blood-vessels is becoming more promi- 



