STRUCTURE AND DEVELOPMENT 59 



like the gill-bars of the fish. In a word, they are the 

 same things. 



The animal feeds and grows during the months of 

 its first summer, and hibernates the following winter; 

 with the warmth of spring it revives and proceeds 

 further along the course of its development. Near 

 the base of the tail two minute legs grow out from the 

 hinder part of the body, and while these are enlarging 

 two front legs make their appearance a little behind the 

 gills. The tadpole now rises more frequently to the 

 surface where it takes small mouthfuls of air. Mean- 

 while great changes are effected inside the body where 

 the various systems of fishlike organs become remodeled 

 into amphibian structures. A sac is formed -from the 

 wall of the esophagus, and this enlarges and divides 

 to form the two simple lungs. The legs increase in 

 size, the tail dwindles more and more, the gills close up, 

 and soon the animal hops out on land as a complete 

 young frog. From this time on it breathes by means 

 of its lungs instead of gills, even though it returns to 

 the water to escape its foes, to seek its prey, and to 

 hibernate in the mud of the lake bed during the winter 

 months. 



All these changes are familiar and natural, but until 

 science places them and similar facts in their proper 

 relations their significance is lost to us. The tadpole is 

 essentially a fish in its general structure and mode of 

 life, even though its heritage is such that it can develop 

 into a higher animal. When it does become a frog it 

 proves beyond a doubt that there is no impassable 

 barrier between fishes and amphibia. Our earlier com- 

 parison of the structures of these two classes of verte- 



