296 HEREDITY IN THE PROTOZOA 



of raising the temperature of cultures containing 

 certain flagellates. In the course of seven years 

 he succeeded in producing a strain that could live 

 at a temperature of 70° C, although at first the 

 entire culture would be killed at a temperature 

 of 61° C. 



Recently, Jollos ('13-'14) caused Paramecium 

 caudatum to acquire a resistance to compounds of 

 arsenic. At first the individuals were killed when 

 1.1 parts of a standard solution were added to 100 

 parts of water. Gradually they became accustomed 

 to live in 5 parts to 100. For several months they 

 retained their acquired resistance when returned to 

 their original environment, but they lost it at last. 

 The loss was slow in fission lines, but sometimes 

 sudden after a conjugation period. Jollos calls this 

 sort of an induced change, ''Dauer-modification/' 

 or long-persisting modification. He believes that it 

 has nothing to do with a nuclear change in the 

 hereditary constitution; i.e., it is not a mutation 

 at all but is probably a change in the plasma or in 

 the macronucleus. The latter is absorbed during 

 conjugation, also during parthenogenesis, hence the 

 sudden disappearance of the Dauer-modification at 

 this time. On the other hand, certain kinds of 

 these modifications may even survive a conjugation 

 period. For instance, after the changes induced by 

 calcium treatment or by high temperature treat- 

 ment the effects may survive for a long time if only 

 fission takes place; they may survive several 

 parthenogenetic periods, or even a conjugation 



