368 ELEMENTS OF GENERAL SCIENCE 



with the eggs. One of them may enter an egg and unite with 

 it, thus fertilizing it. The fertilized frog's egg resembles the 

 spore of the pond scum in that it is a single cell formed by 

 the union of two cells. 



376. Development of the egg. When the egg of the frog 

 has been fertilized it possesses, like the spore of the pond 

 scum, the power of growing into a new organism similar to 

 that which produced it. The first indication of development 

 in the frog's egg is its division into two cells. These again 

 divide, forming four cells, and the process is repeated until 

 the single large egg cell has become a mass of many small 

 cells. This mass of cells elongates ; mouth, eyes, and tail 

 appear ; and the young animal wriggles out of the inclosing 

 jelly. Soon it has the complete form of the familiar tadpole. 



377. The tadpole. The tadpoles are the young of frogs, 

 toads, or closely related animals. The very numerous, small, 

 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 undersurface 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 

 transferred to an aquarium, it is possible to follow the 

 various stages of development at school or at home. 



