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 over the 
edge, and the bony scales in the skin, standing up on 
end, have become teeth. 
And now to our bird. In the embryo chick four 
gill-arches are at first distinguishable, 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- 
Fre. 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 
