DEVELOPMENT OF THE FECUNDATED EGG. 50,'t 



jacent to the primary chorion (to the vitelline membrane), 

 and encloses all the appendages (amnion, foetus, umbilical 

 vesicle). This extended envelope then undergoes a peculiar 

 development; pushed gradually against the vitelline mem- 

 brane, it is duplicated (Fig. 137, 2, and Fig. 139, 2, dotted 

 lines), induces absorption of the latter, is substituted for 

 it, and thus becomes the most external fold of the egg; 

 in its turn it presents little protuberances, and so forms the 

 secondary chorion (Fig. 140, 2'). This second chorion is no 

 more vascular than the first; up to this time the foetus 

 obtains its nutrition from the maternal organism only by 

 imbibition, or receives it by means of the nutritive provision 

 of the yellow (umbilical vesicle). But the development of 

 the second chorion allows the establishment of a definite 

 centre of exchange between the mother and embryo, by the 

 formation of the allanto'is, one part of which will form the 

 placenta. 



Thirdly, the allanto'is is a pouch or protuberance from the 

 inferior part of the intestinal canal (see p. 139, a/, and Fig. 

 1*24, p. 458). When this pouch appears (Fig. 139, at), the 

 amniotic cavity is so much developed that it surrounds the 

 entire foetus, and encloses the pedicle of the umbilical vesicle, 



Fig. 140. Umbilical vesicle and development of the allantols.* 



in such a way as to form a cord by which the foetus is sus- 

 pended in the waters of the amnios. The allantoid protu- 

 berance insinuates itself in this cord (Fig. 140, al), pushes 

 through and places itself by the side of the pedicle of the um- 

 bilical vesicle (omphalo-mesenteric duct), and then conies in 



* o, Umbilical vesicle, a/, Allantols. o, Cavity of the amnios. 2', Second 

 choriou. 



