80 



COMPARATIVE PHYSIOLOGY. 



the allantoic, and vice versa ; they are plainly supplementary 

 organs. 



The Allantoic Cavity.— The degree to which the various em- 

 bryonic membranes fuse together is very variable for different 

 groups of mammals, including our domestic species. 



In ruminants, but especially in solipeds, the allantois as it 

 grows spreads itself over the inner surface of the subzonal 



Fig. 81.— Embryo of dog. twenty-five days old. opened on the ventral side. Chest 

 and ventral walls have been removed, a, nose-pits; 6, eyes; c, under-jaw (first 



f [ill-arch); d, second gill-arch; e,f, q, h, heart (e, right,/, left auricle; g, right, /t, 

 eft ventricle); i, aorta (origin of); kkj liver (in the midale between the two lobes 

 is the cut yelk-vein); I, stomach; m, intestine; », yelk-sac; o, primitive kidneys; 

 p,allantois; g, fore-limbs; A, hind-limbs. The crooked embryo has been stretched 

 straight. (Haeckel, after Bischoff.) 



membrane, often spoken of as the "chorion," while it also 

 covers, though capable of easy detachment, the outer surface of 

 the amnion ; and thus is formed the allantoic cavity. The por- 

 tion of the allantois remaining finally within the fcetus becomes 

 the bladder, which during embryonic life communicates by its 

 contracted portion {urachus) with the general amniotic cavity. 



