246 



A STUDY OF VERTEBRATES 



present on the outside of the body. Soon after, the external 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 completed 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 trans- 

 formation from the tadpole to the young frog is complete. In 

 the green frog and bullfrog the metamorphosis is not completed 

 until the beginning of the second summer. The large tadpoles 

 of such forms bury themselves in the soft mud of the pond bottom 

 during the winter. 



Shortly after the legs appear, the gills begin to be absorbed, and 

 lungs take their place. At this time the young animal may be 

 seen coming to the surface of the water for air. Changes in the 

 diet of the animal also occur ; the long, coiled intestine is trans- 

 formed into a much shorter one. The animal, now insectivorous 

 in its diet, becomes provided with tiny teeth and a mobile tongue, 



instead of keeping the 

 horny jaws used in 

 scraping off algae. After 

 the tail has been com- 

 pletely absorbed and 

 the legs have become 

 full grown, there is 

 no further structural 

 change, and the meta- 

 morphosis is complete. 

 Development of 

 Birds. The white of 

 ^ the hen's egg is put on 



At the left is a hen's egg, opened to show the embryo during the passage of 



at the center (the spot surrounded by a lighter ^e real egg (which IS 

 area). At the right is an English sparrow one 



day after hatching. in the yoke Of yellow 



