VIII 



ALLANTOIS 



473 



end (Figs. 239, 240), and serving for the reception of the renal secre- 

 tion. The study of sections shows that the allantois is simply a 

 pocket of the ventral wall of the gut towards its hind end — corre- 

 sponding exactly with the bladder of an Amphibian. It is thus lined 

 with endoderm and covered externally with splanchnic mesoderm. 

 The allantois like the bladder of the Amphibian bulges into the 

 splanchnocoele. As development goes on the allantois, distended 

 with fluid, increases in size, projecting on the right or upper side of 

 the embryo till it comes in contact with the inner surface of the v 

 somatopleure (Fig. 215, A, all), and with still further growth flattens 

 out against the somatopleure taking a somewhat mushroom-like 

 shape. In the case of an independently living animal such as an 



ilk 



Fig. 215a. — Diagram illustrating the arrangement of amnion, allantois, etc., in the Fowl. 



(After Lillie, 1908.) 



C, twelfth day. alb, albumen ; all, allantoic cavity ; y, yolk. 



Amphibian the allantoic outgrowth of the gut can only increase in 

 size within the restricted space of the splanchnocoele which is already 

 occupied by the viscera. In the Bird embryo on the other hand 

 there are available for the growth of the allantois the wide-spreading 

 extensions of the coelome, on the one hand between amnion and 

 serous membrane and on the other over the surface of the yolk. The 

 allantois accordingly spreads out all round towards the limits of this 

 space (Fig. 215, B). As it does so it loses its rounded vesicular form, 

 its proximal (Fig. 215, B, 2) and distal walls (Fig. 215, B, 1) approach- 

 ing one another. The mesoderm covering its outer surface tends to 

 undergo secondary fusion with that of neighbouring structures. Thus 

 about the end of the sixth day it fuses with the adjacent surface of the 

 amnion. Again towards the time of hatching a similar fusion takes 

 place with part of the yolk-sac. The most important of these fusions 



