REPRODUCTION 



165 



gelatinous envelope. Both nuclei divide twice and produce four nu- 

 clei, three of which degenerate. The two daughter cells, each with one 

 haploid nucleus, undergo paedogamy and the resulting individual 

 now contains a diploid nucleus. 



In Paramecium aurelia, Diller (1936) found simple fragmentation 

 of the macronucleus which was not correlated with any special 

 micronuclear activity and which could not be stages in conjugation 

 or autogamy. Diller suggests that if conjugation or autogamy is to 

 create a new nuclear complex, as is generally held, it is conceivable 

 that somewhat the same result might be achieved by 'purification 

 act' (through fragmentation) on the part of the macronucleus itself, 

 without involving micronuclei. He coined the term hemixis to in- 

 clude these reorganizations. 



Fig. 83. Mitotic and meiotic micronuclear divisions in conjugating 

 Didinium nasutum. (Prandtl, modified), a, normal micronucleus;b, equa- 

 torial plate in the first (mitotic) division; c, anaphase in the first division; 

 d, equatorial plate in the second division; e, anaphase in the second 

 (meiotic) division. 



Meiosis. In the foregoing sections, references have been made to 

 the divisions which the nuclei undergo prior to sexual fusion or con- 

 jugation. In all Metazoa, during the development of the gametes, 

 the gametocytes undergo reduction division or meiosis, by which the 

 number of chromosomes is halved; that is to say, each fully mature 

 gamete possesses half (haploid) number of chromosomes typical to 

 the species (diploid). In the zygote, the diploid number is reestab- 

 lished. In the Protozoa in which sexual reproduction occurs during 

 their life-cycle, meiosis presumably takes place to maintain the con- 

 stancy of chromosome-number, but the process is understood only 

 in a small number of species. 



In conjugation, the meiosis seems to take place in the second 

 micronuclear division, although in some, for example, Oxytricha 

 fallax, according to Gregory, the actual reduction occurs during the 

 first division. Prandtl (1906) was the first to note a reduction in 

 number of chromosomes in the Protozoa. In conjugating Didinium 

 nasutum (Fig. 83), he observed 16 chromosomes in each of the 



