KEPKODUCTION IN PLANTS AND ANIMALS 275 



jet-black ones are toad tadpoles. All of them resemble fish 

 more closely than they do frogs and toads, so far as appear- 

 ance and manner of life are concerned. The tadpoles live in 

 the water all the time and breathe by means of gills, as fish 

 do, while the mature animals live upon land much of the 

 time and breathe by means of lungs. 



During the hatching period the developing tadpoles have 

 taken no other food than that which was stored in the egg. 

 When they begin to swim about they become voracious feeders, 

 eating minute animals and great quantities of green slime, 

 pond scums, and other minute plants. They grow rapidly, 

 and legs soon appear from the under surface of the body. At 

 the same time the tail decreases in size. The gills disappear 

 also, and lungs are formed. Finally, the tadpole, with a form 

 approaching that of the frog but with some remnants of the 

 tail and gills, creeps out upon the land. 



If eggs of the frog or toad are collected carefully and trans- 

 ferred to an aquarium, it is possible to follow the various stages 

 of development at school or at home. 



302. Reproduction in birds and mammals. In most of the 

 higher animals the fertilized egg proceeds, immediately after 

 fertilization, to the development of the beginning stages, or 

 embryo, of the adult animal. The exception to the last state- 

 ment is found in the case of those animals which lay an egg 

 that is incased in a more or less heavy coating, like the eggs 

 of birds and turtles, in which the real egg is a cell similar to 

 the one already described. In these cases there is, in addition 

 to the real egg, a large amount of food material, all of which 

 is incased in the covering. In turtles this covering is a thick, 

 tough, leathery wall ; in birds it is thick and fragile, though 

 it varies in its strength in different species of birds. The egg 

 of the bird undergoes a period of incubation ranging from 

 fourteen or even fewer days, in the case of the smaller birds, 

 to four weeks or more, in the case of turkeys, geese, etc. It 

 is possible to discover the stages in the development of the 



