480 DEVELOPMENT OF THE CHICK. 



The second and third pouches (first and second branchial) 

 open to the exterior for a few hours, but subsequently close up 

 and disappear. The fourth pouch (third branchial) does not 

 open to the surface of the neck and disappears. 



These pouches correspond to the spiracle and first three 

 gill-pouches of the dog-fish, but are at no time respiratory 

 organs. 



The oesophagus, trachea, and lungs begin their develop- 

 ment at this period ; but the alimentary tract is still a straight 

 tube, the middle portion of which is open to the yolk-sac. 



The formation of the tail-fold causes the terminal portion 

 of the gut to become tubular in contrast to the open mid-gut. 

 From this hind-gut there develops on the third day a pouch- 

 like outgrowth the * allantois,' which ultimately expands 

 to form a stalked bladder-like sac, which extends through 

 the ccelom between the amniotic folds over the surface of the 

 embryo and yolk-sac, and comes into close contact with 

 the shell. The allantois is comparable with the bladder of 

 the frog, but is cast off at the time of hatching. It is very 

 highly vascular, and possesses the property of absorbing 

 oxygen from the air-chamber inside the shell and from the 

 atmosphere that penetrates through the pores of the shell. 

 The allantois therefore is the respiratory organ of the embryo, 

 just as the yolk-sac is the alimentary organ until the time of 

 hatching. 



bb. Vascular System. 



The heart on the third day is bent into a S -shaped loop 

 when seen from the left side, with the head pointing to the 

 right. The lower curve of the above S represents the auricle, 

 the upper one the ventricle leading to the truncus. From the 

 truncus six pairs of aortic arches arise and run in the sub- 

 stance of the visceral arches (mandibular, hyoidean, first, 

 second, third and fourth branchial), but the third branchial 

 arch is vestigial. Only three pairs are in existence at a given 

 time. 



These join together on each side of the pharynx, and after 

 supplying the head form the paired dorsal aorta ; and this 



