AN INTRODUCTION TO THE STUDY OF GROWTH. 37 



the production of new cells ceases some time before 

 birth, though in some other mammals it may go on for 

 a longer period, but in all it very early becomes the less 

 significant method of increase. Before attaining any 

 functional importance these newly-formed cells must 

 pass through a long series of developmental changes. 

 Thus in any organism the total number of structural 

 elements is greater, and in early life very much greater, 

 than is the number functionally perfected. Two dif- 

 ferent methods in subsequent development of animals 

 or tissues may be distinguished according to the time 

 relations within which the constituent cells become 

 mature : in the one case the newly-formed cells may 

 change almost simultaneously, or in the other, these 

 changes may be successive in the different cells, so that 

 in the former instance the time interval demanded for 

 the completion of growth is short, in the latter it is long. 

 With these facts in view, an explanation of the growth 

 processes in those animals which early attain a fixed 

 size is to be found in the fact that in them cell develop- 

 ment is condensed into a short period, which in the case 

 of those in which growth is long continued the time 

 during which the development of different cells may 

 occur is increased. Development and the changes 

 involved in growing old are, however, by no means 

 synonymous, so that although in those animals with a 

 fixed size there are always to be found undeveloped 

 cells, yet it is not a correct inference that these cells are 

 also young in the sense that they might still complete 

 their development. It appears, rather, that the capacity 

 for undergoing expansive change is transient, and that 

 those cells which fail to react during the proper growing 

 period of an animal have lost their opportunity for ever. 

 Here again is a difference between the two groups ot 

 animals which have been contrasted, since in those the 



