FROGS AND BIRDS 231 



weed by means of a pair of suckerlike projections ; later a mouth 

 is formed, and the tadpole begins to feed upon algae and other tiny 

 water plants. At this time, about two weeks after the egg was 

 laid, gills are 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. 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. 



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 completely absorbed and the legs have 

 become full grown, there is no further structural change, and the 

 metamorphosis is complete. 



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 transformation 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 frogs bury themselves in 

 the soft mud of the pond bottom during the winter. 



Development of Birds. — The white of the hen's egg is put on 

 during the passage of the real egg (which is on the yolk or yellow 

 portion) to the outside of the body. Before the egg is laid a shell 

 is secreted over its surface. If the fertilized egg of a hen is broker 

 and carefully examined, on the surface of the yolk will be found 

 a little circular disk. This is the beginning of the growth of an 

 embryo chick. If the development is followed in a series of eggs 

 taken from an incubator at intervals of twenty-four hours or less, 

 this spot will be found to increase in size ; and later the little 

 embryo will lie on the surface of the yolk. Still later small blood 

 vessels can be made out reaching into the yolk for food, and the 



