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 



-- the ancestors of birds were once 



/5. PI -^ aquatic and fish-like. But how about 



'i,V '";- ' -^ lungs? Fishes have none, and indeed 



-V- ; I S i^^ their aquatic life such organs would 



/'" V "jt be useless. Nevertheless, as we shall 



^ ; -^^ see, the lungs of reptiles, birds, and 



^: t mammals are legacies from the crea- 



|v ^ V tures of the sea. 



fpi_, 4'.) "^ f :: Many fishes have withm their 



. 't ^1 I ^i» bodies a thin-walled sac, known as 



4 ~\ %\ ^- " the swim-bladder. This is filled with 



:r gas, and as the fish ascends to the 



5 jl ^/%':^-^A surface, or dives to where the pres- 

 "^ %^ /f tS^ sure of the water is ver^' great, the 



J % . :f 1|^ amount of gas varies ; so that the 



M. W specific gravitv of the fish changes 



i I m with that of the water. This swim- 



I bladder is generally connected with 



^^ the throat by a delicate tube; and 



Teonf foWialfowing ^tou- iH 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 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. 



