Development of the Fourth Day 243 



third day has already been described (pages 

 198-203). 



The changes that take place in the arterial 

 system will first be described, and then the 

 changes that occur in the venous system. 

 At the close of the third day, it will be re- 

 membered, three pairs of aortic arches had 

 been formed, from before backwards, lying 

 in the mandibular, hyoid and first viscal 

 folds. 



During the fourth day, two other pairs of 

 aortic arches appear, in the second and third 

 visceral folds. There are thus in the chick 

 five pairs of aortic arches, corresponding to 

 the anterior five of the six pairs found in the 

 tadpole. But in the chick, there are no gill 

 capillaries at any stage of development ; and 

 there is never any distinction between the 

 afferent and efferent branchial vessels, the 

 blood flowing through one continuous vessel 

 from the truncus arteriosus to the dorsal aorta. 

 The condition of the aortic arches in the chick 

 embryo is more comparable to the condition 

 in the frog after metamorphosis. 



The main changes that convert the arteries 

 of the embryo into the condition which they 

 attain in the adult chick are the following: 



