152 PROTOZOOLOGY 



trophozoites, because of the preconjugation fission (d-e). The 

 microiuiclear divisions are similar to those that have been de- 

 scribed for Paramecium caudatum and finally two pronuclei are 

 formed in each conjugant. Exchange and fusion of pronuclei 

 follow. In each exconjugant, the synkaryon divides once to form 

 the micronucleus and the macronuclear anlage (k-l) which de- 

 velop into the "spireme ball" and finally into the macronucleus 

 (m-o). 



A sexual process which is somewhat intermediate between the 

 sexual fusion and conjugation, is noted in several instances. Ac- 

 cording to Maupas' classical work on Vorticella riehulifera, the 

 ordinary vegetative form divides twice, forming four small in- 

 dividuals, which become detached from one another and swim 

 about independently. Presently each becomes attached to one side 

 of a stalked individual. In it, the micronucleus divides three times 

 and produces eight nuclei, of which seven degenerate; and the 

 remaining nucleus divides once more. In the stalked form the 

 micronucleus divides twice, forming four nuclei, of which three 

 degenerate, the other dividing into two. During these changes the 

 cytoplasm of the two conjugants fuse completely. The wandering 

 nucleus of the smaller conjugant unites with the stationary nu- 

 cleus of the larger conjugant, the other two pronuclei degenerat- 

 ing. The synkaryon divides several times to form a number of 

 nuclei, from some of which macronuclei are differentiated and 

 exconjugant undergoes multiplication. 



Another example of this type has been observed by Noland 

 (1927) in Metopus es (Fig. 76). According to Noland, the con- 

 jugants fuse at the anterior end (a), and the micronucleus in each 

 individual divides in the same way as was observed in Para- 

 mecium caudatum (b-e). But the cytoplasm and both pronuclei 

 of one conjugant pass into the other (/), leaving the degenerating 

 macronucleus and a small amount of cytoplasm behind in the 

 shrunken pellicle of the smaller conjugant which then separates 

 from the other (j). In the larger exconjugant, two pronuclei fuse, 

 and the other two degenerate and disappear {g, h). The synkaryon 

 divides into two nuclei, one of which condenses into the micro- 

 nucleus and the other grows into the macronucleus {i-m). This 

 is followed by the loss of cilia and encystment. 



What is the significance of conjugation? What are the condi- 

 tions which bring about conjugation in the ciliates? These are 



