CHAPTER XVII 



GROWTH AND REGENERATION 



When an animal reaches a size that is characteristic for the 

 species it ceases to grow, and it may appear that this happens 

 because the cells of the body have lost the power of further 

 growth. That the cessation of growth is not due to such a loss 

 of power is shown by the ability of many animals to regenerate 

 a lost part. 



In some animals practically all the organs of the body show 

 this remarkable regenerative power, so that there can be no 

 doubt that the cessation of growth is not due to the loss of 

 power of the cells to grow, but rather to something that inhibits 

 their growth. 



One of the most curious facts connected with the regeneration 

 of a part is its rate of growth at different levels. If the tail of a 

 fish or of a salamander is cut off near its base, the new part grows 

 faster than when the tail is cut off nearer to the tip. Moreover, 

 the new part that arises from the basal cut grows more rapidly 

 at first and more slowly later. What is the meaning of this 

 difference in rate? The result cannot be explained as due to 

 differences in the food supply at different levels, for the rate of 

 growth is very httle affected whether the animal is fed or starved. 

 The difference in rate cannot be explained as due to a difference 

 in the material at the two levels, because the material derived 

 from the base, that grows more rapidly for a time, begins also to 

 grow less rapidly as the new part gets longer, and behaves in this 

 respect in the same way as the material derived from nearer the 

 tip. The difference does not seem to be due to the difference in 

 size of the two surfaces, except perhaps at the very beginning, 

 for in other forms, in the earthworm, for example, the same 



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