176 RUTH L. PHILLIPS 



nomena indicated by the actual rate of division may be expressed 

 in other ways, such as the percentage of high divisions above 

 the mean rate for a given period, percentage of divisions below 

 this mean, and the percentage of deaths. It is believed that 

 such expressions of the activities of the animals are helpful 

 in interpreting their reaction to food. These percentages have 

 been computed and are included in the tables. 



One of the newer methods in biometry is the determination of 

 what I have elsewhere termed the 'significance factor.' I have 

 used this term as being less clumsy than any phrase describing the 

 mathematical processes involved, believing that its meaning is as 

 clear as a longer expression. Gross ('20) and MacDowell ('21) 

 have used this method in interpreting physiological results, but, 

 so far as I am aware, it has not previously been used in experi- 

 mental work with the Protozoa. The use of the significance 

 factor has proved to be the most helpful of any instrument em- 

 ployed in interpreting the facts. It has been possible by its 

 \ise to decide as to the relative value of different foods or the 

 effects of changes in medium. Division rate alone, even when 

 aided by percentages of high divisions and the like, fails to reveal 

 the entire truth in such cases. For instance, in dealing with 

 changes in medium, the conclusion from an inspection of these 

 data would be that a moneywort infusion was stimulating when 

 first used in all the lines tested. Determination of the signifi- 

 cance factor reveals that the only real evidence of acceleration of 

 metabolism was in the case of the mixture A'B'C. We may 

 state, therefore, that although the division rate is the truest 

 index of the metabolism we have yet found for the Protozoa, the 

 interpretation of its meaning is greatly helped by other indices, 

 especially by the use of the 'significance factor,' which at once 

 enables one to settle any doubt as to the trend of the metabolism 

 as expressed by the rate of division. 



Under normal conditions Paramecium feeds mainly upon 

 bacteria. The enormous variety of bacteria in infusions makes it 

 possible for this animal to obtain adequate food under natural 

 conditions. However, since Paramecium has no choice as to the 

 type of food, but must ingest whatever comes in its way, it is 



