THE INFUSORIA 449 



cilia and swim off. Each microconjugant attaches itself to a macroconjugant 

 that is to say, to an ordinary sedentary individual ; each of the conjugants 

 has a single micronucleus and macronucleus, but as soon as they become 

 associated the changes preparatory to syngamy begin. In the microconjugant 

 the micronucleus divides three times to produce eight micronuclei. In 

 Carchesiwm the first of these divisions is an equating division ; the second 

 reduces the number of chromosomes from sixteen to eight ; and the third 

 division is again an equating division (Popoff, 125). Meanwhile the macro- 

 nucleus is in process of degeneration, and is breaking up into fragments. 

 Of the eight micronuclei, seven degenerate, one persists and divides into two 

 pronuclei. In the macroconjugant, meanwhile, similar events are taking 

 place, but the micronucleus only divides twice, first by a reducing, then by an 

 equating division, to produce four micronuclei, of which three degenerate, 

 while the fourth persists and divides into the two pronuclei. 



Of the two pronuclei now present in each conjugant, one degenerates in 

 each case ; the persistent pronucleus of the microconjugant passes over into 

 the macroconjugant and copulates with its persistent pronucleus. The frag- 

 ments of the macronucleus also pass over into the macroconjugant, but are 

 there absorbed slowly. The body of the microconjugant then falls off and dies ; 

 only the macroconjugant is fertilized. 



Variations of minor importance are seen in the behaviour of the synkaryon 

 of the exconjugant after fusion of the pronuclei has taken place. For example, 

 in Paramecium bursaria the synkaryon divides to form four nuclei, two of 

 which become macronuclei, whereupon the exconjugant divides into two 

 ordinary individuals (Hamburger, 842) ; in Licnophora the synkaryon divides 

 into eight, which become a micronucleus and a macronuclear chain of seven 

 segments (Stevens, 872) : in Carchesium the synkaryon divides also into eight 

 to furnish a micronucleus and seven separate macronuclei, but the micro- 

 nucleus then divides six times, with subsequent divisions of the body and 

 sorting out of the macronuclei, until seven individuals, each with a single 

 micronucleus and macronucleus, are produced (Popoff, 125) ; in Anoplophrya 

 the synkaryon divides into four nuclei, two of which degenerate, the remaining 

 two becoming a micronucleus and a macronucleus respectively. The method 

 of nuclear reconstruction may vary even in the same species, as shown by 

 Prandtl (126) in the case of Didinium. 



The most important deviations from the usual scheme of conjugation are 

 seen in those forms in which there is no separate micronucleus in the ordinary 

 condition. The cases of Opalina and Ichthyophthirius, parasitic forms and 

 therefore open to the charge of degeneration, are dealt with below. In 

 Trachelocerca plioeni copter us, a free-living species, conjugation has been 

 described by Lebedew (93) between individuals containing many nuclei all 

 similar in appearance, each with a large karyosome. Prior to conjugation 

 the chroniatin passes out of the karyosome into the nuclear cavity of each 

 nucleus (Fig. 188, A, B), which then divides into four. The chromatin forms 

 a compact mass at one pole of each nucleus. During conjugation these masses 

 of chromatin pass out of the nuclei, and lie free in the cytoplasm between 

 them (Fig. 188, C (?) ; each such mass is now to be regarded as a micro- 

 nucleus and lies in a clear area, finally becoming a vesicular nucleus with a 

 distinct alveolar structure ; the old nuclei can now be considered as macro- 

 nuclei. All the nuclei now collect in a mass near the middle of the body. 

 The macronuclei ultimately degenerate ; the micronuclei multiply by fission, 

 but ultimately, according to Lebedew, they all degenerate with the exception 

 of one in each conjugant ; the persistent micronucleus divides into two pro- 

 nuclei which conjugate in the usual way ; unfortunately, the author's observa- 

 tions contain so many gaps that this statement cannot be considered estab- 

 lished so decisively as could be desired. The exconjugants contain each a 

 single synkaryon which divides by successive divisions into a number of nuclei 

 not differentiated into micronuclei and macronuclei. 



The case of Trachelocerca, as it is described, furnishes an important clue 

 to understanding the origin of the heterokaryote condition of Infusoria from 



29 



