GILLS IN THE YOUNG NECTURUS. 175 



pattern of the growth changes in the normal gill were repeated 

 in the regenerating gill. 



If we compare the rate of growth in the regenerating gill with 

 the rate of the normal gill we find that the stage reached in nor- 

 mal growth in sixteen days is reached in regeneration in about 

 eighteen days. 



That the pattern of the growing gill is not retraced by the 

 regenerating has been established beyond question in these ex- 

 periments. In the regeneration of the foot of the young Ncct/tnis 

 the course of normal development is pretty accurately repeated 

 and the same is known to be true of many other forms. 



Previous experiments by others seem to indicate that if wide 

 variations occur in normal growth we should expect to find like 

 wide variations in regeneration. 



One pattern of gill is as efficient for respiration as another pro- 

 vided it posesses the same number of filaments. The same is 

 true of the regenerated gill. The normal pattern of the foot is 

 duplicated in regeneration because the type evolved is that best 

 adapted to the needs of the animal. 



If these considerations be well founded we are led to regard 

 physiological efficiency as the important factor in the regenera- 

 tion of the gills. 



In other words we may conclude that functional and structural 

 regeneration may run parallel or they may follow widely diverg- 

 ing lines. 



