466 ZOOLOGY FOR MEDICAL STUDENTS chap. 



During the fourth day the allantois makes its appearance. At this 

 time the splanchnopleure extends downwards over the surface of the 

 yolk. Along the mid-line of the embryo, immediately under the noto- 

 chordj a groove-like portion is tucked off from the rest. This groove is 

 continued forwards and backwards as a blindly ending tube. The whole 

 forms the rudiment of the alimentary canal (Fig. 195, A, ent) which 

 therefore at this stage consists of three portions — an anterior tubular 

 fore-gut, a posterior tubular hind-gut and an intervening mid-gut 

 which opens freely below into the cavity filled with yolk — the yolk- 

 sac (Fig. 195, A, y). The allantois arises as a pocket - like median 

 downgrowth of the floor of the hind-gut (splanchnopleure) near its 

 hinder end (Fig. 195, C, all). It becomes conspicuous during the 

 fourth or fifth day as a rounded bladder projecting into the wide 

 coelomic space. In a six-day egg the allantois has increased much in 

 size and is coming in contact with the inner surface of the somatopleure 

 (Fig. 195, D, all). Against this with further growth it flattens itself, 

 taking on a mushroom-like shape and extending outwards all along its 

 edge in close apposition to the somatopleure (Fig. 195, E). The somato- 

 pleure is now in close contact with the shell membrane over a great part 

 of its extent, what is left of the diminishing albumen lying down in the 

 lowest part of the cavity of the egg shell. The mesoderm covering that 

 part of the allantois lying next the somatopleure has meanwhile become 

 richly vascular, being supplied by a large allantoic artery (Fig. 196, B, a.a) 

 and its blood draining into an allantoic vein {a.v) on each side. 



This highly vascular outer wall of the allantois, which eventually as 

 the chick increases in size comes to lie in close apposition to the inner 

 surface of the shell, separated from it only by the very thin intervening 

 layer of somatopleure, is the respiratory organ of the developing chick 

 until it takes its first breath of air into its lungs just before hatching. 

 The blood-vessels seen branching over the inner surface of the pieces of 

 egg-shell from which a bird has hatched are those of the allantois, for it 

 is a remarkable fact that the allantois, like the amnion and the greater 

 part of the somatopleure, is of use only in the developing embryo and is 

 discarded at hatching. 



It should not be forgotten that the primary function of the allantois 

 is that of a urinary bladder. In a terrestrial animal with its egg enclosed 

 in a hard shell there is no possibility of the poisonous secretion of the 

 kidneys diffusing away into the surrounding medium and consequently 

 it is necessary to keep it stored in one spot where it will not injure the 

 tissues in general. 



The development of the Fowl affords a good example of the lower 



