AMPHIBIANS 



281 



single laying. Immediately before leaving the body of the female 

 they receive a coating of jellylike material, which swells up after 

 the eggs are laid. Thus they are protected from the attack of fish 

 or other animals which might use them as food. The fertilized 

 egg soon segments (divides into many cells) , and in a few days, if the 

 weather is warm, these cells have grown into an oblong body which 

 shows the form of a tadpole. Shortly after the tadpole wriggles out 

 of the jellylike case and begins life outside the egg. At first it 

 remains attached to some waterweed by means of a suckerlike 

 projection; later a mouth is formed at this point and the tadpole 

 begins to feed upon algae or other tiny water plants. At this 

 time gills are present on the outside of the body. Soon after this, 

 the external gills are re- 

 placed by gills which grow 

 out under a fold of the skin 

 which forms an operculum 

 somewhat as in the fish. 

 Water reaches the gills 

 through the mouth and 

 passes out through a hole on 

 the left side of the body. 

 As the tadpole grows larger, 

 legs appear, the hind legs 

 making their appearance 

 first, although for a long time 

 locomotion is performed by 

 means of the tail. In some 

 species of frogs the changes 

 from the egg to adult are 

 completed in one summer. 

 A month or two after hatching, the tadpole begins to eat less, 

 the tail is used up rather rapidly (being absorbed into other parts 

 of the body), and before long the transformation from the tad- 

 pole to the young frog is complete. In the green frog and bull- 

 frog the metamorphosis is not completed until the beginning of 

 the second summer. The large tadpoles of such forms bur>' them 

 selves in the soft mud of the pond bottom during the winter. 



Frog's eggs from three to ten hours old. All 

 stages from four cells to thirty-two cells may 

 be noted. From photograph, enlarged four 

 times, by Davison. 



