The Life-History of Hypotrichous Infusoria. 625 



in which it is possible to know with any degree of certainty exactly 

 what the pedigree of the subjects of the experimentation is, and 

 unless one has the daily record of the ancestry of each protozoon 

 and knows its status in the life-cycle, any results obtained lose a 

 large part of their value. Nothing emphasizes this point more 

 forcibly than the record of my experiments with the dibasic 

 potassium phosphate. 



VII. EFFECT OF LIGHT ON THE RATE OF DIVISION. 



Maupas ('88) made some interesting experiments on the effect 

 of light on the division-rate of various Infusoria, by keeping 

 cultures for one month in the light and then for one month in the 

 dark and then comparing the rate of division during the two 

 periods. But it would seem that his method is open to criticism 

 for it is clearly impossible to keep the conditions absolutely con- 

 stant during the two months of the experiments, not to mention 

 the fact that, according to Maupas himself, ''senescence" is 

 increasing. Consequently it is impossible to say that the dif- 

 ference, or absence of difference, in the rate during two consecu- 

 tive months shows the effect, or non-effect, of light on bipartition. 

 I would call attention to the fact that he found less difference in 

 light and darkness than my records show for any two consecutive 

 months of any of the cultures when light and all other factors 

 have been apparently constant. 



With this in mind I made an experiment on the effect of light 

 on the division-rate of Oxytricha fallax, and endeavored to elimi- 

 nate the factors which seem to vitiate Maupas's experiments. 

 This was accomplished by isolating an individual from each line 

 of Oxytricha A-culture, and starting with them a second culture 

 (designated A^) in absolute darkness.^ By this method the light 

 and dark series were carried on simultaneously and this ruled out 

 the question of relative "senescence"; and at the same time varia- 

 tion in the food was reduced to a minimum, since the same 

 infusion was supplied to both cultures simultaneously. Tempera- 

 ture differences were avoided also. It would seem, therefore, that 

 light was the only factor removed in the case of culture AS and 



^The culture was necessarily, of course, subjected to light for two or three minutes each day when 

 the record of divisions was being taken. 



