TUB DEVELOPMENT OF VEIllEBllATA. 



259 



amnion eventually becomes completely separated from the 

 true amnion, by absorption of the connexion between them, 

 and by the time that the splitting of the mesoblast has ex- 

 tended right round the ventral side of the yolk-sac, it forms 

 a thin membrane completely separated from everything else, 

 and coming to lie close under the egg-membrane when the 

 albumen has all been absorbed. The portion of it which 

 does not come from the amniotic fold but is split off the 

 yolk-sac is called the serous membrane, but that and the 

 false amnion form a single membrane in the end. It now 

 forms a sort of extra egg-membrane, and may be disregarded 

 after this stage. 



COXQ 



CCCLOM 



Fig. 187. TRANSVERSE SECTION OF CHICK, END OF SECOND DAY OF INCUBATION. 



11. The Allantois. The necessity for the nutrition of 

 the growing embryo is met by a very early development 

 of blood-vessels in the area opaca as it spreads over the 

 yolk. The relations of these with the vascular system in 

 the embryo itself will be described later. But respiration 

 soon becomes as great a necessity as nutrition. The tad- 

 pole's internal and external gills would be useless to the 

 chick, which does not live in the water. Some other means 

 of respiration is needed, and this is provided by an out- 

 growth from the posterior end (cloaca) of the mesenteron, 

 called the allantois (figs. 133 and 135). This is nothing else 

 than the frog's urinary bladder, enlarged and adapted to a 

 new function. It consists, of course, of hypoblast and 

 splanchnic mesoblast, the latter layer containing abundant 



