CONDITIONS FOR CONJUGATION 369 



fission, and resembling each other closely in size and appearance, 

 there are those which conjugate repeatedly and those which do 

 not, and the same may be said for the slowly dividing strains. 



But it seems probable, in the light of our experimental knowl- 

 edge, that there is an indirect connection between conjugation 

 and fission-rate. Whenever conjugation occurs in a culture 

 en masse, it usually does so after a period of accelerated division. 

 This relation has been emphasized by several investigators, 

 working with diverse genera of Infusoria, both free living 

 and parasitic. In fact, Maupas regarded it as one of the primary 

 conditions preceding a conjugation epidemic in the many species 

 which he studied, and applied it practically in his work. 



In further support of this view — that conjugation is conditioned 

 by a preliminary period of increased division — we may recall 

 the effects produced by adding salts to the cultures of Par. 

 aurelia when renewed. In a medium of sodium nitrate (of 0.001 

 to 0.005M concentration) conjugation is almost invariably 

 augmented in this species (cf., however, race i 6 to the contrary, 

 which conjugated as well in an aqueous medium). Oftentimes 

 conjugation can be elicited in such 'salt cultures' when none 

 occurs in the controls renewed with an aqueous medium. It 

 may be shown very easily by means of slide cultures, changed 

 daily, that NaNOs in such concentrations acts as a stimulant 

 to growth. The division rate in these treated lines may be main- 

 tained at a considerably higher level than that of the controls 

 for at least two weeks (table 8). Some races apparently do 

 not tolerate the continuous action of the salt and die after two 

 or three days of accelerated growth, and the death rate is greater 

 in any race so treated than in the controls. These observations 

 apply to races of both species of Paramecium, but only occa- 

 sionally is a race of caudatum found which will tolerate the action 

 of the salt as used in these experiments. 



J. Spek ('19) has conducted similar experiments on Par. 

 caudatum, using various salts in the medium, and finds that 

 many of these tend to accelerate fission-rate. 



My experiments upon Paramecium using MgS04 in the culture 

 medium indicate that this salt has a depressing effect upon 



