102 Viewpoints in the Study of Growth [Jan. 



question whether the size of an organism depends upon the size of 

 its component parts — its cells, — or upon their number. Does an 

 organism grow by increase in the number of new cells or by an 

 enlargement of the old ones ? In other words, is there a relation be- 

 tween body size and cell size? It would take us too far afield to 

 examine in detail the evidence bearing on this and related morpho- 

 logical questions of the nature of growth. They have been vigor- 

 ously debated. The answers probably differ for unlike species and 

 certainly for different tissues. If in the lower forms of life differ- 

 ences in ultimate body size are due in the main to differences in cell 

 number, the cell size being approximately constant, it by no means' 

 follows that this Statement holds throughout the progressive stages 

 of growth in higher organisms where other factors than cellular 

 increase and enlargement also are involved. Perhaps we may dis- 

 tinguish between the processes of tissue dififerentiation and growth 

 by reference to the changes in the number and the size of the cells 

 respectively. Much has been made of the necessity of maintaining 

 a certain proportion between the size of the nucleus and that of the 

 cytoplasm composing the body of the cell. Doubtless physiological 

 functions are closely associated with the presence of the nucleus 

 and it appears that when a cell is to attain a very large size it is 

 almost always found to contain several nuclei. Indeed, as we shall 

 see, Minot has presented the relative increase in the cytoplasm with 

 accompanying differentiation as the proximate cause of senescence. 

 Experience shows that there is a fairly definite upper limit in 

 size which the individuals of any species rarely exceed. There are 

 forms for which the variations may be very wide (as has already 

 been pointed out) ; and it is reported that some of the lower forms, 

 e. g., actinians, can be caused by suitable feeding to reach a colossal 

 size far beyond that which they ordinarily attain in nature. To the 

 mammalian species with which we are primarily concerned here, 

 however, this does not apply except in the limited degree determined 

 by heredity. Why the body size is thus fixed is not known. The 

 fact of its invariable character makes it possible to apply quantita- 

 tive methods to the study of growth with some confidence in the 

 consequences which are to be expected from any normal procedure, 

 and with some appreciation of the Standard to which proper growth 

 should conform and by which all deviations are to be measnred. 



