Rates of Growth of Individuals 35 



toplasmic bulk) was logarithmic; in other words new 

 bulk was added to the entire population in proportion 

 to the bulk already present at any one time. 



Under conditions either of size modification with 

 age of culture or of slow multiplication, therefore, the 

 increase of total bulk must be the criterion of accumu- 

 lation of living substance, because accumulation is no 

 longer sensibly proportionate to the increase in num- 

 ber of individuals. 



Comparison of results. It is far from certain that 

 the growth curves for individual unicellular organisms 

 are similar in all species. It is doubtful whether the 

 curves are the same for all races or under all condi- 

 tions which allow of growth, even in the single species 

 Paramecium caudatum. Whether the increase in size 

 is uniform in rate or not, is important for ascertaining 

 the nature of the regulation of growth. Upon the 

 basis of Jennings's curve for Paramecium it might be 

 said that increase of size was cyclic, and that the cycle 

 of size-increments coincided, perhaps causally, with 

 the cycle of mitoses. Upon the basis of most of the 

 other studies reviewed above, it would be said that the 

 increase of size was nearly uniform; that fission was a 

 subsidiary occurrence, coming at whatever time the 

 body volume was doubled; and that its occurrence did 

 not essentially influence the processes of volume in- 

 crease. 



3. Various Influences upon Growth in Size 



The relative effects of diverse circumstances should 

 help in the analysis of the processes concerned in vol- 

 ume increase, however meager the data may be. 



Nutrition. That food is necessary for increase in 

 size might be thought to be axiomatic. The evidence, 

 however, is, in part, to the contrary. Most of the data 

 upon Paramecium, especially those of Jennings ('08b), 



