External Factors and Size 147 



ences result also from changes in the chemical nature 

 of the medium ? A culture medium is not an easy ob- 

 ject of chemical analysis. It is simpler and more satis- 

 factory to make up new medium with definite differ- 

 ences from the old, and find the result of the transfer. 

 To do this it is necessary to use large volumes of med- 

 ium, or else very frequent replacements of medium. 



Concentration of salts. Most studies of the effect of 

 osmotic pressure or total concentration have been car- 

 ried out by placing comparable samples of organisms 

 into various dilutions of a salt solution. 



Paramecium. The effects, upon the rate of growth 

 of individual Paramecia, of various concentrations of 

 sodium chloride were tested by Estabrook (TO), with 

 results which have already been given in figure 18. In 

 concentrations above M/30 the bodies were smaller 

 at all ages than the controls in hay infusion. At con- 

 centration M/10 growth apparently ceased. The ques- 

 tion naturally arises whether the smaller sizes in M/10 

 solutions were due to an inhibition of assimilation or 

 due to a reduced water content. 



The answer to this is apparently given by tests 

 which Estabrook made upon adult Paramecium. 

 When, as shown in figure 49, individuals were put into 

 M/10 NaCl, they shrank markedly during the first 15 

 minutes. Then they began to recover in volume, 

 though probably not to the original value. This is the 

 sort of progression of body volume which occurs in 

 response to transfer to a similar solution on the part 

 of frogs and most other aquatic organisms which have 

 been tested. Curiously enough, this was earlier stated 

 by Enriques ('02) to be the course of events on the part 

 of certain protozoa; yet it is not recorded that he ac- 

 tually made any measurements of size. 



To lower concentrations of sodium chloride, the re- 

 sponse was apparently similar but of less degree. Fig- 



