THE AM N ION AND ALLANTOIS. 585 



which is now inclosed within the body of the embryo, the in- 

 testinal canal is developed. 



Thus by the constriction which the fold of germinal mem- 

 brane, in which the abdominal walls are formed, produces at 

 the umbilicus, the body of the embryo becomes in great meas- 

 ure detached from the yelk-sac or umbilical vesicle, though 

 the cavity of the rudimentary intestine still communicates with 

 it through the vitelline or omphalo-mesenteric duct, and con- 

 tains part of the yelk-substance with which the vesicle was 

 filled. The yelk-sac contains, however, the greater part of the 

 substance of the yelk, and furnishes a source whence nutriment 

 is derived for the embryo. In birds, the contents of the yelk- 

 sac afford nourishment until the end of incubation : but in 

 Mammalia, the office of the corresponding umbilical vesicle 

 ceases at a very early period, the quan- 

 tity of yeik is small, and the embryo FIG. 2-20. 

 soon becomes independent of it by the 

 connections it forms with the parent. 

 Moreover, in birds, as the sac is emptied, 

 it is gradually drawn into the abdomen 

 through the umbilical opening, which 

 then closes over it : but in Mammalia it 

 always remains on the outside ; and as 

 it is emptied it contracts (Fig. 220), 

 shrivels up, and together with the part 



of its duct external to the abdomen, is 



detached and disappears either before, Human embryo with um _ 

 or at the termination of intra-uterine biiicai vesicle; about the 

 life, the period of its disappearance vary- fifth week (after Daiton . 

 ing in different orders of Mammalia. 



When bloodvessels begin to be developed, they ramify 

 largely over the walls of the umbilical vesicle, and are actively 

 concerned in absorbing its contents, and conveying them away 

 for the nutrition of the embryo. 



The Amnion and Allantois. 



At an early stage of development of the foetus, and some 

 time before the completion of the changes which have been 

 just described, two important structureSj called respectively 

 the amnion and the allantois, begin to be formed the amnion 

 being developed by the external, and the allantois by the in- 

 ternal layer of the blastodermic membrane. 



The amnion is produced in the following manner: The ex- 

 ternal layer of the blastodermic membrane is raised up in the 

 form of a fold around the body of the embryo, so that the 



