C DOBELL 135 



of ordinary individuals'. They grow and divide subsequently in the 

 ordinary fashion (§ 3). 



6. In Vorticella and its allies, conjugation of a different type 

 occurs^ The two conjugating individuals are of different size — a large 

 and a small, respectively sessile and motile. As in the previous case, 

 they come in contact and fuse — their meganuclei sooner or later 

 degenerating. The micronucleus in the larger individual divides twice 

 successively by mitosis, giving rise to four nuclei, of which three 

 degenerate and disappear. In the smaller individual the micronucleus 

 undergoes three successive divisions, thus producing eight nuclei, of 

 which seven degenerate. The two conjugating individuals now fuse 

 completelyl Their two remaining nuclei divide once more, one of the 

 two daughter nuclei so formed in each degenerating, the other persisting. 

 The latter then come together and fuse. The single uninucleate in- 

 dividual resulting from conjugation then reorganizes its nuclear system 

 in a manner comparable with that described above (§ 5) — the details of 

 the process probably being different in different forms'". 



7. A " reduction " or halving of the chromosomes (meiosis) has been 

 shown to occur during the micronuclear divisions which take place at 

 the beginning of conjugation ^ The halving appears to occur usually 



' The details of the process whereby the new nuclear apparatus is constructed after 

 conjugation may be very different from those outlined above. In all cases, however, the 

 reorganization is of essentially the same nature — the formation of a new meganucleus and 

 micronucleus from the single fusion nucleus. 



- This account is based upon V. vionilata (Maupas, 1889). 



3 According to Maupas (1889) the membrane clothing the body of the smaller in- 

 dividual is cast off and degenerates — it is not absorbed into the body of the larger. 



* The accounts given above are conformable with the views of nearly all those workers 

 who have studied the conjugation of the ciliates since the time of Maupas. It should be 

 stated, however, that this accepted version of the sequence of nuclear events has been 

 questioned by some workers, Hoyer (1899) believed that in Colpidium no nuclear fusion 

 occurs during conjugation — the migratory nuclei are simply exchanged, and the stationary 

 nuclei then degenerate. The new nuclear apparatus is therefore formed, in each in- 

 dividual, entirely from the nucleus which it receives from its partner. More recently, 

 Dehorne (1911, 1911a) has stated that the same thing happens in Puramecium, and that 

 he has confirmed Hoyer's account of Colpidium. Dangeard (1911, 1911a) has contradicted 

 this in the case of the latter form (wrongly called Colpoda in his first paper), and main- 

 tains that the accepted version is correct. I believe that Hoyer and Dehorne are wrong in 

 their contention. Maupas, R. Hertwig, Calkins and Cull, and all other recent workers 

 who have studied conjugation, are unanimously agreed that a nuclear fusion does occur. 



' Chromosome reduction has not been shown to occur in Paramecium. Calkins and 

 Cull (1907) believe that the chromosomes are halved at the first micronuclear division, 

 but they are "far too numerous to count." They judge them to be approximately 165 in 

 number at this stage, whilst at other divisions the number is "greater than this." In 

 Stentor (2 species) the chromosomes appear to be reduced to a quarter — not a half — of the 

 usual number during the "reduction" divisions (Mulsow, 1913). 



