THE INFUSORIA 387 



On the other hand, Joukowsky has recently failed to induce conjugation 

 by hunger in Pleurotricha after experimenting for eight months and 

 reaching the 458th generation, and Calkins has cultivated Paramoecium 

 caudatum to the 620th generation, without conjugation, by changing the 

 food when the periods of depression set in. 



There is some reason to believe that the onset of the epidemic of 

 conjugation in certain CILIATA is associated with a material diminution 

 in size ; the largest specimens of a species having the power or vital 

 force to divide by fission, do not need the stimulus of conjugation for 

 further reproduction. 



In the long series of varieties that the process of conjugation 

 presents in the HETEROKARYOTA, attention may be called to 

 three conditions which, when separated from the series, appear to 

 present essential differences. In most of the free-swimming CILIATA 

 all the individuals of a swarm in which there is an epidemic of 

 conjugation are of the same size and structure, and it is probable 

 that any one individual can conjugate with any other. There is 

 no distinction of sex whatever. 



In the conjugation of Spirochona two individuals apparently 

 identical, situated close to one another on the same gill of 

 Gammarus, bend towards each other and conjugate by their oral 

 discs. Subsequently one of the two individuals separates from the 

 gill and becomes absorbed (partially or wholly) by the other one, 

 and from that moment it ceases to exist as an individual. In this 

 case no external features of sexual differentiation have been 

 observed, and perhaps do not exist, between individuals that are 

 capable of conjugating with one another ; but after conjugation has 

 begun the important difference between the one that absorbs, which 

 has been compared to an ovum of the metazoa, and the one that is 

 absorbed, which has been compared to a spermatozoon, is exhibited 

 (cf. p. 393). 



In the third case, as exhibited by . Vorticella and some other 

 PERITRICHA, the difference between individuals that can conjugate 

 is well marked long before conjugation actually occurs (Figs. 43 and 

 76). The stalked form, which may be called the female individual, 

 is not capable of conjugating with another individual of the same 

 kind, 1 nor, on the other hand, are the small free-swimming forms 

 that are periodically produced the males capable of conjugating 

 with one another. The only way in which conjugation can be 

 effected is by one female individual conjugating with one male 

 individual. In other words, the differentiation of sex appears to be 

 complete in the case of Vwticdla and some other forms. 



During the process of conjugation very important and complicated 

 changes occur in the nuclei of .both individuals, and in all probability 



1 It is possible that in some species a conjugation of the females may occur (see 

 Plate Ixxiii. Fig. 96 of Biitschli's " Infusoria "'). 



