THE NUCLEI OF UROLEPTUS MOBILIS 335 



It is obvious, in these cases, that the multinucleate phase is 

 an adaptation from the uninucleate condition, and the conclu- 

 sion may be drawn that the independent nuclei, by fusion, return 

 to a more primitive and more generalized uninucleate condition 

 prior to division. 



In some forms, finally, the independent nuclei do not fuse 

 again prior to division and each divides independently, the prog- 

 eny passing to the daughter cell in which they happen to be 

 (Trachelocerca, Dileptus gigas, Ichthyophthirius, Opalina). 



h. Absence of fusion of micronuclei during division. The num- 

 ber of micronuclei in Uroleptus mobilis varies between two and 

 six. At no time during vegetative life, division, or conjugation is 

 the number reduced to one, except for the very brief period during 

 conjugation, when the functional pronuclei fuse, and before the 

 first division of the synkaryon. Even at this time, however, 

 there are still from three to five degenerating pronuclei in the 

 cytoplasm of each individual. During the five days required 

 for reorganization of the cell after conjugation, there are two 

 micronuclei, and they begin to divide in the same period as the 

 new macronucleus. As a result of their division, the fully reor- 

 ganized cell has six to seven micronuclei. During the early 

 phases of division, viz., during the fusion of the macronuclei, 

 these micronuclei are found in full mitosis (figs. 4 to 14). I have 

 never seen as many as six in mitosis at one time, but have fre- 

 quently found individuals with five and four at this period of 

 macronuclear concentration. When the macronucleus is ready 

 to divide I have found individuals with two and with four micro- 

 nuclei in mitosis (figs. 10 and 11), but never with more. If two 

 is the initial number as indicated by ex-conjugants, then it is 

 reasonable to infer that the number becomes reduced to two 

 before division of the macronucleus. If this is the case, how 

 has the reduction taken place? On this point I have no certain 

 evidence. 



Balbiani ('60, '61) at first believed that multiple micronuclei 

 are bound together in a common pouch with fine connectives like 

 those between multiple macronuclei. No later observer has 

 confirmed the hypothesis, and Balbiani himself retracted this 



