188 Regulation of Size in Unicellular Organisms 



nucleus. The sizes are both regulated, but that both 

 are controlled by the same factors is as likely as that 

 one is a factor in the regulation of the other. 



Neither the nucleus nor the cytoplasm is homo- 

 geneous; each contains many structures and sub- 

 stances. In each structure the chief substance is 

 water; a measurement of size is mainly a measure- 

 ment of the relative amount of water present. It is 

 unlikely that any other substance is always present in 

 the same proportion to water throughout growth and 

 all other variable circumstances. Growth or other 

 change in any other one substance may or may not 

 involve a proportional change of volume. The same 

 difficulty is present in all studies of size; it is possible 

 that size is a mere resultant of the constancy or fluctu- 

 ation of many constituents which in turn are primarily 

 the regulated quantities. 



Size of some other structures. The nucleus, when 

 present, is the most conspicuous of the differentiated 

 structures of a unicellular organism. In most it is the 

 only structure which can be measured with accuracy. 

 In a few isolated instances other structures have been 

 correlated in size with body size. Thus in Difflugia, 

 Jennings ('16) found that spine size and spine num- 

 ber were unrelated to body size and to nuclear size. 

 But in Arcella, Hegner ('19a) found spine number 

 positively correlated with body size within a clone. 

 In Ameba the size which contractile vacuoles attained 

 before emptying was correlated positively with the 

 size of the nuclei, according to Botsford ('26). Com- 

 parisons of this kind are required in great variety be- 

 fore factors effective in regulating structural size can 

 be sifted out. 



