4 GARY N. CALKINS 



in a previous publication. As in previous work, the number of 

 divisions was recorded each day for each of the ex-conjugants. In 

 addition to the ex-conjugants, one individual from the original 

 culture which had not conjugated, was isolated at the same time 

 to serve as a sort of control. This line was designated X; the others, 

 A-B, C-D, E-F, G-H, etc. 



The early history of the division rate for all of these forms is 

 given in the accompanying table. 



The interest in the present paper centers in the first pair, A-B, 

 for it was in this line of cultures that the aurelia form appeared. 

 B died after a few days, but A recovered from the shock of reor- 

 ganization and soon began to divide at a slow rate. As is the cus- 

 tom in such work, one of the daughter-cells from an early division 

 was killed and stained, to determine if conjugation had been nor- 

 mally completed. The preparation shows that, instead of reor- 

 ganizing with one micronucleus characteristic of P. caudatum, this 

 individual had two, but otherwise appeared normal. From time 

 to time after this individuals from this line of cultures were fixed 

 and stained, and these preparations give a good history of the 

 nuclear conditions at different periods. Some of these specimens 

 are shown in photographs on Plate i. Fig. i represents an individ- 

 ual in the third generation after conjugation, with four micronuclei 

 (already divided for the ensuing generation) and the as yet incom- 

 plete macronucleus. Fig. 2 represents an individual in the 24th 

 generation; Fig. 3, one in the 43d; Fig. 4, one in the 46th; Fig. 6 

 shows two micronuclei in the end stage of division, the daughter- 

 halves are connected by a delicate thread not seen in the photograph ; 

 Fig. 7, finally, represents an individual in the 220th generation after 

 complete recovery of the P. caudatum condition. The change from 

 the one form to the other occurred during the first two weeks in May. 

 Figs. 3, 4, and 5 represent three individuals from the same ancestor 

 in the 43d generation. The first two individuals have each two 

 micronuclei, the third has only one. All of these micronuclei show 

 a marked increase in size over those shown in Fig. 2 for example, 

 representing an individual in the 24th generation. 



During the month of May and until three months after the cul- 

 ture was started, individuals appeared here and there with but one 



