PHYSIOLOGICAL VARIATIONS IN PARAMECIUM 471 



C. Vitality tests 



Vitality of the different pure lines is tested from time to time 

 by the division rate method. Two representative individuals of 

 each pure line are isolated in small flat-bottomed watch glasses, 

 containing four to five drops of twenty-four hour hay infusion, 

 a single individual in each glass. These are examined, the num- 

 ber of divisions noted, and one individual is selected and isolated 

 daily in fresh twenty-four hours hay. Each test extends over a 

 period of thirty days, the division rate is then averaged for both 

 individuals in all the lines and for consecutive periods of five days 

 each. This gives a fairly accurate basis for comparison of the 

 relative vitality of progeny from different quadrants (p. 476). 



D. Conjugation tests 



It is a well-recognized fact at the present time that a sudden 

 change from conditions inducing rich feeding and rapid multi- 

 plication to conditions involving a cessation of growth and mul- 

 tiplication will cause Paramecium to conjugate provided the 

 power to conjugate is in them. Jennings, Zweibaum, Woodruff, 

 and others have employed this method with success, each work- 

 ing it out in his own way. For years at Columbia we have been 

 in the habit of obtaining conjugations when desired by transfer- 

 ring Paramecium from rich laboratory cultures to small Syracuse 

 dishes 65 mm. in diameter and 5 mm. deep with a capacity of 

 16 cc. The bacteria transferred with the animals are soon ex- 

 hausted and if the Paramecia will conjugate at all they will do so 

 within forty-eight hours. If the bacteria are not exhausted a state 

 of equilibrium is established and the ciliates may live for weeks 

 without conjugating, multiplying slowly. The same method 

 is employed in making the conjugation tests in these experi- 

 ments, but several steps are necessary. The tests are made once 

 a month and the first step is the preparation of a rich culture. 

 For this the contents of the permanent cultures are distributed 

 in three or four vials where the organisms are allowed to multi- 

 ply for two days. They are then used for seeding rich food media 



