370 HOYT S. HOPKINS 



growth. Most races become readily adapted to a 0.01 M 

 concentration, and may continue to live for months in such 

 cultures to which hay has been added. The division rate is 

 not appreciably diminished by this salt, although extensive 

 experiments may yield more definite results than I have thus 

 far attained. In the few conjugation experiments in which 

 MgS04 was used (P. aurelia) the results were negative, except 

 when the solution was used indirectly to increase the osmotic 

 concentration of the medium in a derived culture. These 

 cultures, when renewed' with such a medium, continued to live, 

 but failed to grow with sufficient rapidity to develop the initial 

 impulse to conjugate (ex hypothese). 



Let us now consider the second condition, or antecedent, to 

 conjugation, namely, that of subjecting the cultures to a period 

 of dormancy. From what we know concerning the relation of 

 fission-rate to conjugation, it might be conjectured that such a 

 period of rest (dormancy) renders the organisms so treated 

 potentially more capable of rapid division than those which had 

 been dividing previously. Or, conversely stated, a regulation in 

 division-rate may occur in organisms subjected to long-continued 

 multiplication, so that they do not divide so fast as formerly. 

 Such a regulation has been shown by Jollos ('13) to occur in 

 paramecia subjected to a higher temperature. Lines which had 

 been growing at 19°, after being transferred to 31°, divided at a 

 rate of 9 per forty-eight-hour period; after six months at this 

 temperature, however, at the rate of 7. 



If any regulation in division-rate does occur in organisms sub- 

 jected to nutritive conditions favoring rapid growth, one ought 

 to be able to test it experimentally, using two cultures of the 

 same strain, one of which had lain dormant for several months, 

 the other having undergone continuous growth. Accordingly, 

 such an experiment was tried. A race of organisms (P. caudatum 

 39) in which conjugation had never been observed was chosen 

 in order to eliminate any secondary effects which might accom- 

 pany this process. The culture 39A had been maintained under 

 rather uniform conditions of growth, renewed with hay infusion 

 every two or three weeks for a period of about six months, 



