EFFECT OF EXCRETION PRODUCTS OF PARAMAECIUM 569 



the experiment, tended to render the media of the hnes different, 

 this variation was ehminated by cross infections of all the lines 

 of all the cultures daily at the time of isolation. It is believed 

 then, that every practicable precaution was taken to ensure as near 

 absolutely the same food conditions as it is feasible to obtain, 

 and consequently that the results have not been influenced by varia- 

 tions in nutrition. 



It is not apparent that the consistent variations in the division 

 rate in the different volumes of medium can be the result of the 

 products of metabolism of bacteria, in view of the great care 

 taken to maintain identical flora in all the preparations. It is 

 possible, however, that slight irregularities in the curve may be 

 explained on this basis and this point will be mentioned later. 



The remaining factor to be considered is the possible effect of 

 the excretion products of the paramaecia themselves. That the 

 infusoria in their ceaseless activity are continually voiding ap- 

 preciable amounts of carbon dioxide and other products of me- 

 tabolism is evident from the work of previous investigators. The 

 question then is — is the amount excreted sufficient to affect the 

 division rate appreciably under the conditions of the experiments? 

 Taking the average rate of division during the experiments as 

 one and one-half divisions per day, the average number of or- 

 ganisms in a preparation during the first twenty-four hours would 

 be one and one-third— one during the first sixteen hours, and 

 two during the following eight hours. However improbable it 

 may seem that this number of individuals can excrete sufficient 

 toxic substances to have an appreciable effect on the division rate 

 when diluted, for example, by twenty drops or by forty drops,^^ 

 I believe the experiments outlined make this highly probable, 

 and point to the conclusion that the variations in the daily division 

 rate in the different volumes of water is due to the excretion products 

 of the paramaecia themselves. 



^* With the division rate increasingly more rapid with increase of volume, there 

 would actually be more excretion products in the larger volumes than in the smaller. 

 This however can be disregarded. 



