282 H. S. JENNINGS 



II. METHODS 



Most of the experiments followed a general plan, the chief 

 features of which may be here set forth. The object was, to 

 compare, under similar conditions, a set of the animals that had 

 conjugated with another set that was ready to conjugate, but 

 was prevented from doing so. Both Paramecium caudatum and 

 Paramecium aurelia were employed in the work. Abundant con- 

 jugation was obtained in the way described by Maupas ('89), 

 Calkins and Cull ('07), and others. In the evening large numbers 

 of the animals were taken from the large cultures and placed 

 in watch glasses; early the following morning they were usually 

 beginning conjugation. Paramecium caudatum is especially 

 favorable for obtaining with certainty the first stages of the 

 process, since as Maupas ('89, pp. 171, 182) has noted, this 

 animal conjugates in the early morning, commencing at about 

 five o'clock. If therefore there were no conjugations when the 

 watch glasses were set, one can be certain that any pairs found 

 early the following morning have just united. 



Split pairs. At the beginning the pairing animals fit loosely 

 together; they at first, as a rule, adhere together only by their 

 anterior ends. At such a time it is easy to separate them, by 

 drawing them repeatedly into a fine pipette. The separated 

 individuals are then isolated and cultivated separately. 



Pairs. Other pairs are allowed to complete conjugation. 

 They separate spontaneously after about twelve hours; the two 

 members are then isolated and cultivated separately, under the 

 same conditions as the members of the 'split pairs.' 



In this way two sets are obtained, taken from the same cul- 

 ture, both ready to conjugate and beginning the process at the 

 same time; the only difference between them lies in the fact that 

 one is allowed to complete the process, while the other is not. 

 By cultivating the two sets under identical conditions it becomes 

 possible to determine what difference is made by conjugation. 



Designation. The terms 'pairs' and 'split pairs' will be used 

 in referring to the members of the two sets, and to their progeny. 

 The two members of any pair, or of any split pair, will be desig- 



