i 7 8 



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 



the ancestors of birds were once 

 aquatic and fish-like. But how about 

 lungs? Fishes have none, and indeed 

 in their aquatic life such organs would 

 be useless. Nevertheless, as we shall 

 see, the lungs of reptiles, birds, and 

 mammals are legacies from the crea- 

 tures of the sea. 



Many fishes have within their 

 bodies a thin-walled sac, known as 

 the swim-bladder. This is filled with 

 gas, and as the fish ascends to the 

 surface, or dives to where the pres- 

 sure of the water is very great, the 

 amount of gas varies; so that the 

 specific gravity of the fish changes 

 with that of the water. This swim- 

 bladder is generally connected with 

 the throat by a delicate tube; and 



FIG. 133. Lung of Chame- , , i_ xi 



Icon, foreshadowing con- in these tWO Structures W6 have the 

 dition in bird. , , / ,LI i i i i 



homologues 01 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. 



