114 



The Bird 



upon us: the jaw of a shark is nothing but a greatly 

 changed gill-arch, which has doubled up, bent forward 

 and hinged to the skull. The skin has grown o\er the 

 edge, and the bony scales in the skin, standnag up on 

 end, have become teeth. 



And now to our bird. In the embryo chick four 

 gill-arches are at first distniguishable, but these soon 

 begin to alter their position, to fade away, or to change 

 in some way, and in our bony skull we may trace them 

 as follows (see Fig. 89). The upper half of the first gill- 



FiG. 88. — Lower mandible, tongue, and hyoid bones of Bald Eagle. 



arch forms the bones of the upper jaw, palate, jugal, and 

 quadrate, and the lower jaw completes the entire arch. 

 The central part of the second gill fades into nothing, 

 but the top is present as the columella-bone of the ear, 

 while the base is transformed into the head and two 

 blunt barbs of the arrow-like bone of the tongue. The 

 two long barbs of this bone correspond to the third gill 



