THE VERTEBRATE ANIMALS 



289 



posited at a single laying. Imuu'diately before leaxiuK the body 

 of the female they recei\'e a coating of jellylike material, which 

 swells up after the eggs are laid. Thus thej- are protected from 

 the attack of fish or other animals which might use them as food. 

 The upper side of the egg is dark, the light-colored side being 

 weighted down with a supply of yolk (food). The fertilized egg 

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

 weather is warm, these cells have each 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 

 water weed by means of a 

 pair of suckerlike projec- 

 tions; later a mouth is 

 formed at this point, and 

 the tadpole begins to feed 

 upon algaj or other tiny 

 water plants. At this time, 

 about two weeks after the 

 eggs were laid, gills are pres- 

 ent on the outside of the 

 body. Soon after, the ex- 

 ternal gills are replaced 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 

 first, although for a time locomotion is performed by means of the 

 tail. In the leopard frog the change from the egg to adult is com- 

 pleted in one summer. In late July or early August, the tadpole 

 begins to eat less, the tail becomes smaller (being absorbed into 

 other parts of the body), and before long the transformation from 

 the tadpole to the young frog is complete. In the green frog and 

 bullfrog the metamorphosis is not completed until the begin- 

 ning of the second summer. The large tadpoles of such forms 



HUNT. ES. BIO. 19 



Frogs' eggs from three to ten hours old. All 

 stages from four cells to thirty-two cells may- 

 be noted. Photograph, enlarged four times, 

 by Davison. 



