382 GENERAL ZOOLOGY 



from a culture of this protozoan are kept under continuous 

 observation, it is found that at fairly regular intervals binary 

 fission takes place and a single individual divides into two 

 daughter cells. Each of the latter grows and in about 10 hours 

 attains the size-limit characteristic of the species, when division 

 again occurs. A large number of binary fissions may take place 

 after this fashion; but sooner or later, under ordinary condi- 

 tions in a laboratory culture, an entirely different sort of 

 phenomenon, known as conjugation, occurs. In this process 

 two individuals touch, at first in front, and then along the 

 entire surface of one side, so that the cytostomes come together; 

 the macronucleus swells and breaks up, the fragments even- 

 tually dissolving; and the micronuclei by two successive divi- 

 sions produce eight nuclei in each conjugant. Seven of these 

 nuclei disintegrate while the eighth divides, forming a stationary 

 micronucleus and a migratory micronucleus. Each migratory 

 micronucleus then passes into the opposite cell and fuses with 

 the stationary micronucleus to form a synkaryon or fertilization 

 nucleus. The conjugants now separate and in each the 

 synkaryon divides twice, producing four micronuclei, two of 

 which are transformed into macronuclei. The two remaining 

 micronuclei each divide again, accompanied by a division of the 

 cell, so that two complete individuals, each provided with a 

 macronucleus and two micronuclei are derived from each of the 

 conjugants (Fig. 214). 



Woodruff has shown that if the medium in which isolated 

 paramecia are living is kept fresh by constant changing, con- 

 jugation does not occur for thousands of generations (12,000, in 

 the period from 1907 to 1921). It was found, however, that 

 every 40 or 50 generations the macronucleus degenerates and is 

 replaced by chromatin from the micronucleus, a process known 

 as endomixis. The nuclear changes, as may be seen from the 

 figure, are similar to those occurring in conjugation, except 

 that there is no exchange of nuclear material between two 

 individuals; i.e., reciprocal fertilization does not occur (Fig. 215). 

 The common feature of both conjugation and endomixis, viz., 

 the periodic replacement of the macronucleus with material 

 from the micronucleus, would seem to have some significance as 

 a means of rejuvenescence. However, it is generally believed, 

 in view of experiments with other species of Protozoa, that 



