2 Regulation of Size in Unicellular Organisms 



organism is confined is invisible, for its boundaries and 

 its skeleton are also changing. It is independent of 

 any particular structural constituent and maintains its 

 individuality in a medium composed of the same sub- 

 stances as itself. Driesch ('98) suggested that the ac- 

 curacy with which size is controlled by metabolizing 

 organisms proved the presence of a sort of regulation 

 which could not be performed in the absence of special 

 vital properties. 



Regulation. The concrete resultant of the various 

 processes which produce the constancy of size in an 

 organism, and among organisms, is not unique. Every 

 characteristic of a living being represents the manifes- 

 tation of similar regulations. Each specific structure, 

 each mode of physiological response, each activity, is 

 a product of accurately measured functions. But the 

 regulation of size is certainly as universal as, and per- 

 haps more universal than, any other function that has 

 yet been measured in living organisms. 



The characteristic called size has not been ignored 

 by students of life; there have been numerous dis- 

 courses upon the theoretical importance of it, and upon 

 the physical forces probably involved in its control. But 

 experimental work has been infrequent and of an iso- 

 lated character. Nevertheless, for many kinds of or- 

 ganisms there exist data which, though having been 

 recorded chiefly for other purposes, yet deserve to be 

 correlated with one another, toward the end of finding 

 what factors are directly concerned in the regulation 

 of size. The cultivation of this field of science-in-the- 

 making will undoubtedly result in a yield that will be 

 significant in proportion to the intensity of the cul- 

 tivation. 



In the present review it is proposed to discover what 

 conclusions can be drawn regarding the everyday 

 methods by which organisms regulate their sizes, from 



