446 APPLIED BIOLOGY 



remain throughout life, functioning as described above. In 

 all amphibia (frogs and salamanders) gill-slits are found in 

 the embryos and are commonly present in the young tadpoles ; 

 but in all adult frogs and toads and in most salamanders the 

 gill-slits are closed when the adult stage is reached, and then 

 they breathe by lungs and skin. Finally, in all reptiles, 

 birds, and mammals gill-slits develop in the embryos, but 

 they normally close before hatching or birth. They are never 

 in these animals of any possible use for breathing, for in the 

 stage in which the gill-slits occur the embryos do not live 

 in water, where gills could serve for breathing. 



It is clear why gill-slits develop in fishes and amphibia 

 which use them as respiratory organs; but obviously this 

 does not explain their presence in the higher vertebrates. 

 Nor has any other physiological explanation been found; 

 and the gill-slits appear to be useless structures. Why then 

 do they develop in the embryos of all reptiles, birds, and 

 mammals ? The only answer which is satisfactory to modern 

 zoologists is that gill-slits in higher vertebrates suggest that 

 these had fish-like ancestors in the far-distant ages, and that 

 from these ancestors the gill-slits have been inherited. In 

 short, gill-slits in reptiles, birds, and mammals are ancestral 

 reminiscences. 



There are many other structures in embryos of higher 

 forms which have been explained only on the ground of 

 inheritance. 



365. Bird Development. All species of birds are ovip- 

 arous (external development) ; and the eggs require in- 

 cubation. For this a certain temperature is essential (about 

 103 F. for hen's eggs). The eggs are fertilized soon after they 

 leave the ovary and enter the oviduct, and cell-division goes 

 on for about a day while the eggs are passing through the 

 duct to the exterior. But soon after an egg is " laid " it 

 becomes cooled to below the normal temperature and devel- 

 opment stops. Within a variable number of days, the de- 



