718 



OF REPRODUCTION. 



should also be provided, for unloading the blood of the carbonic acid with 

 which it becomes charged during the course of its circulation. The tempo- 

 rary Respiratory apparatus now to be described, bears a strong resemblance 

 in its own character, and especially in its vascular connections, with the gills 

 of the Mollusca; which are prolongations of the external surface (usually 

 near the termination of the intestinal canal), and which almost invariably 

 receive their vessels from that part of the system. This apparatus is termed 

 the Jlllantois. It consists at first of a kind of diverticulum or prolongation of 

 the lower part of the Digestive cavity, the formation of which has been already 

 described. This is at first seen as a single vesicle, of no great size (Fig. 

 285, g] ; and in the Foetus of Mammalia, which is soon provided with other 

 means of aerating its blood, it seldom attains any considerable dimensions. 

 In Birds, however, it becomes so large as to extend itself around the whole 

 Yolk-sac, intervening between it and the membrane of the shell ; and through 

 the latter it comes into relation with the external air. The preceding diagram 

 (Fig. 286) will serve to explain its origin and position in the Human ovum. 

 The chief office of the Allantois in Mammalia is to convey the vessels of the 



c 



Diagram of Human Ovum, at the time of formation of Placenta; a, rnuco-gelattnous substance, block- 

 ing up os uteri ; 6, 6, Fallopian tubes; c, c, Decidua vera, prolonged at c'2. inio Fallopian tube ; rf, cavity 

 of uterus, almost completely occupied by ovum; e, e, angles at which Decidua vera is reflected ;./", De- 

 cidua serotina ; g, allanlois; h, umbilical vesicle ; i, amnion ; k, chorion, lined with outer fold of serous 

 tunic. 



