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 
Fia. 133.—Lung of Chame- 
leon, foreshadowing con- 
dition in bird. 
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 
in these two structures we have the 
homologues of 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 cesophagus, 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. 
