THE INFUSORIA 



389 



in the cytoplasm as well. For the elucidation of these changes it 

 is convenient to consider, in the first place, the phenomena that 

 have been observed in certain HOLOTRICHA (cf. Fig. 37). When 

 two individuals have effectively conjugated, the micronucleus of 

 each swells up and undergoes division by mitosis into two micro- 

 nuclei (II., III.). This is immediately followed by a division into 

 four (IV., V.). Of these four nuclei three degenerate and are 

 either absorbed or rejected 

 from the body (VI., and ,////. 



Fig. 38). The remaining 

 one which may be called 

 the sexual nucleus under- 

 goes another division into 

 two nuclei. One of these 

 crosses the line of junction 

 of the conjugating in- 

 dividuals and enters the 

 cytoplasm of the other in- 

 dividual, and may be called 

 the migratory nucleus. 

 The other remains in the 

 cytoplasm of the parent, 

 and may be called the 

 stationary nucleus. Thus, 

 as a result of the divisions 

 of the original micro- 

 nucleus of each of the 

 conjugating individuals, 

 five nuclei are formed : three degenerate and disappear, one migrates, 

 and the remaining one is stationary. A fusion of the migratory 

 nucleus of one individual and the stationary nucleus of the other 

 then takes place to form the cleavage nucleus (VIII., IX.), and soon 

 after this has occurred the two individuals separate. The cleavage 

 nucleus soon divides into two (X.), and generally a second time 

 into four, or a third time into eight. Ultimately, however, one of 

 the halves of a division gives rise to the new meganucleus, and the 

 other to the micronucleus of a daughter individual. The new 

 meganucleus can therefore be traced back to its origin from micro- 

 nuclear elements, and to this there is no exception. The mega- 

 nucleus of a conjugating individual never gives rise to the mega- 

 nucleus of an individual that has been released from conjugation. 



Returning to the earlier stages of conjugation, and tracing the 

 fate of the original meganuclei, we find that, apart from minor 

 changes in the arrangement of the chromatin network, the 

 meganuclei do not seem to be affected by the union of the two 

 individuals. Later on, however, they become irregular in outline 



Fio. 38. 



A stage in conjugation of Colpidium colpoda just before 

 the formation of the migratory and stationary micro- 

 nuclei. M, meganucleus ; m\, the sexual nucleus ; m^, 

 the three nuclei undergoing degeneration. (After 

 Hoyer.) 



