RESPIRATION OF EMBRYO; ALLANTOIS. 



461 



oviparous animal ; and it serves for the aeration of its fluids, up to the 

 time when it quits the egg. In the ovum of the Mammal, the chief 

 office of the Allantois is to convey the vessels of the embryo to the cho- 

 rion ; and its extent bears a pretty close correspondence with the extent 

 of surface through which the chorion comes into vascular connexion with 

 the decidua, this extent varying considerably in the different orders of 

 Mammalia. Thus in the Carnivora, whose placenta extends like a band 

 around the whole ovum, the allantois also lines the whole inner surface 

 of the chorion, except where the umbilical vesicle comes into contact 

 with it. On the other hand, in Man and the Quadrumana, whose pla- 

 centa is restricted to one spot, the allantois conveys the foetal vessels to 

 one portion only of the chorion (Fig. 146) ; although, according to Coste, 



Fig. 146. 



Diagram of the Circulation in the Human Ovum, at the commencement of the formation of the Placenta: 

 a, venous sinus, receiving all the systemic veins; b, right auricle; o', left auricle; c, right ventricle; d, 

 bulbus aorticus, subdividing into e, e', e", branchial arteries; /, arterial trunk formed by their confluence; 

 g, vena azygos superior; h, confluence of the superior and inferior azygos;./, vena cava inferior; k, vena 

 azygos inferior ; m, descending aorta ; n, n, umbilical arteries proceeding from it ; o, umbilical vein ; q, 

 omphalo-mesenteric vein; r, omphalo-mesenteric artery, distributed on the walls of the vitelline vesicle t ; 

 v, ductus venosus; y, vitelline duct; z, chorion. - 



it completely surrounds the embryo. When these vessels have reached 

 the Chorion, they ramify in its substance, and send filaments into its 

 villi ; and in proportion as these villi form that connexion with the 

 uterine structure which has been already described ( 811), do the ves- 

 sels increase in size. They then pass directly from the foetus to the 

 chorion ; and the allantois, which is no longer of any use, ceases to 

 present itself as a distinct sac. The lower part of it, however, pinched 

 off (as it were) from the rest, remains as the Urinary bladder ; and the 



