206 



PROTOZOOLOGY 



without involving micronuclei. He coined the term hemixis for this 

 reorganization. 



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 of 

 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. 



Fig. 91. 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. 



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. 91), he observed 16 chromosomes in each of the 

 daughter micronuclei during the first division, but only 8 in the 

 second division. Since that time, the fact that meiosis occurs during 

 the second micronuclear division has been observed in Chilodonella 

 uncinata (Enrique; MacDougall), Carchesium polypinum (Popoff), 

 Uroleptus halseyi (Calkins), etc. (note the ciliates in Table 5 on p. 

 168). In various species of Paramecium and many other forms, the 

 number of chromosomes appears to be too great to allow a precise 

 counting, but the observations of Sonneborn, as quoted elsewhere 

 (p. 234) and of Jennings (1942) on P. aurelia and P. bursaria respec- 



