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PROTOZOOLOGY 



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

 conjugation. 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 number (haploid) of 

 chromosomes typical to the species (diploid). In the zygote, the 



Fig. 79. Diagram showing the endomixis in Paramecium aurelia 

 (Woodruff), a, normal individual; b, degeneration of macronucleus 

 and first micronuclear division; c, second micronuclear division; d, 

 degeneration of 6 micronuclei; e, cell division; f, g, first and second 

 reconstruction micronuclear division; h, transformation of 2 micro- 

 nuclei into macronuclei; i, micronuclear and cell division; j, typical 

 nuclear condition is restored. 



diploid number is reestablished. In the Protozoa in which sexual 

 reproduction occurs during their life-cycle, meiosis presumably 

 takes place to maintain the constancy 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 



