178 The Bird 



We have learned that the chick in the egg passes 

 through a stage when it possesses several well-de- 

 veloped gills. This proves that in the dim, distant past 

 a-. the ancestors of birds were once 



A. ^H ^ aquatic and fish-like. But how about 



;f; -'^v-r"'^:^ lungs? Fishes have none, and indeed 

 vv : >: ;j in their aquatic life such organs would 



r' ; - '"^ be useless. Nevertheless, as we shall 

 1^ • see, the lungs of reptiles, birds, and 



# mammals are legacies from the crea- 



i^;^ ^^ V "^ tures of the sea. 



sf - . ■ • 



^■. . , X ■ ... 



f^^ ^ ; Many fishes have withm their 



-■"^y -%l 11 bodies a thiin-walled sac, known as 



y ■ % : ^ the swim-bladder. This is filled with 



:; gas, and as the fish ascends to the 



-I iy^i-pi surface, or dives to where the pres- 



. i%^y^^ J ^M sure of the water is very great, the 



"*/ %^ H^ amount of gas varies; so that the 



1% ^ specific gravity of the fish changes 



i 4 ^ with that of the water. This swim- 



®| bladder is generally connected with 



^ the throat by a delicate tube; and 



Fig. 133. — Lune of Ohame- . ,1 , , , 1 ,1 



leon, foreshadowing con- m these two structures we have the 



dition in bird. , , r j.i i • 1 > i 1 



homologues oi the birds lungs and 

 trachea. Proof of this is to be found in the growth of the 

 lungs in all young chicks. A tiny bud appears upon the 

 primitive oesophagus, just behind the little gill-clefts, and 

 increases in size until it is larger than the food-canal itself. 

 It then in turn divides into two equal parts which become 

 diminutive flaps, or canals — the beginnings of the lungs. 



