Development of the Third Day 191 



As in the case of the frog, the most an- 

 terior, and first formed gill cleft is called the 

 Jiyomandibular cleft, while the others, from 

 before back, are the first, second and third 

 gill clefts. In the frog, it will be remembered, 

 there were five pairs of visceral clefts. The 

 fate of the visceral clefts will be discussed 

 a little later. 



The parts of the side walls of the pharynx 

 between the gill clefts, and also the anterior 

 border of the hyomandibular cleft and the 

 posterior edge of the third or last cleft, 

 become somewhat swollen and rounded, and 

 are known as \h& gill arches. (Figs. 63, A B, 

 72). As in the frog, again, the first arch is 

 known as the mandibular, the second arch as 

 the hyoid, and the other arches as theyfri-/, 

 second, and third. 



In development and structure, then, the 

 visceral arches and clefts of the chick and 

 frog are essentially the same, except for 

 the presence, in the frog, of an extra pair 

 of arches and clefts, and of the gills which 

 border the visceral arches of the frog, but 

 are not present at any stage in the develop- 

 ment of the chick. The presence in the chick 

 of these fish-like though, to it, functionless 



