WHAT EVOLUTION IS 31 



lungs when the tadpole approximates 

 the state of the frog. But before this 

 metamorphosis has taken place the 

 tadpole, in structure and in activities, 

 recalls in many important particulars 

 the state of a fish. 



Examples of this kind may also be 

 found in the course of human develop- 

 ment. When the human embryo is a 

 small fraction of an inch in length a 

 definite number of narrow transverse 

 clefts appear on its neck as shown 

 in the uppermost figure on page ^2>- 

 These clefts lead into the throat and 

 correspond in position to the gill open- 

 ings of fishes. Moreover the sup- 

 ports between the clefts, the arches, 

 which are numbered in the figure, 

 carry large arteries resulting from 

 the division of the main blood-vessel 

 that emerges from the embryonic 

 heart, just as the gill arches of fishes 

 are supplied by large vessels from the 



