Ejfect of Chciuicals on Groiuth 491 



MATERIAL AND METHODS 



In all the experiments one pure line of Paramecium has been 

 used, that designated by Jennings^ as L • Thiswas the largest race 

 he isolated, measuring from 200 to zjomicrons, orabout one-fifth 

 of a millimeter. All the specimens used are descended from a 

 single pair of conjugants which he selected from a wild culture. 

 Thus, by using animals all derived from a single pair, any possible 

 differences due to diverse ancestry are excluded and only variation 

 within a pure line has to be dealt with. The Paramecia were kept 

 in stock in large flat dishes holding about 2 to 3 liters. The hay 

 infusion in which they were kept was made fresh about once a 

 week, by pouring off some old infusion and adding new hay and 

 fresh water. The Paramecia were thus kept in good condition 

 and afforded an abundance of material at all times. 



Paramecia undergoing division were picked out from the stock 

 culture with the capillar\' pipette, and put on a slide with a con- 

 cave depression. At the moment when the two halves separated, 

 one of the daughter cells was placed in a few drops of hav infusion 

 in one concavitv on a ground glass slide, the other daughter cell 

 was placed in a similar concavity on the same slide, this latter 

 depression containing the solution whose effect was to be studied. 

 The time at which the halves were isolated was noted, and after 

 a certain interval they were taken out, killed and measured 

 separately. This gives a direct method of measurmg growth, as 

 the specimens, can be killed at any age desired. The control in 

 hay infusion gives a normal curve of growth and acts as a check, 

 while the other shows directly the effect of the solution upon the 

 normal growth. The fact that the two halves come from the 

 same mother cell gives an almost ideal condition of experimenta- 

 tion, since the two organisms studied are of the same age, their pre- 

 vious history is the same, and they are in the same physiological 

 condition at the moment of separation. 



The Paramecia were taken out by means of the capillary pipette, 

 gathered into one or two drops of liquid and then killed by sud- 



* Loc. cit., page 494. 



