REPRODUCTION 



207 



tively, indicate clearly the occurrence of meiosis prior to nuclear ex- 

 change during conjugation. 



Information on the meiosis involved in the complete fusion of gam- 

 etes is even more scanty and fragmentary. In Monocystis rostrata 

 (Fig. 92), a parasite of the earthworm, Mulsow (1911) noticed that 



f ' ^^V 



m 



-m 





^tzm&j^ 



w 





Fig. 92. Mitosis and meiosis in Monocystis rostrata (Mulsow). a-g, 

 mitosis; h-j, meiosis. a, a resting nucleus in the gametocyte; b, develop- 

 ment of chromosomes; c, polar view of equatorial plate; d, longitudinal 

 splitting of eight chromosomes; e, separation of chromosomes in two 

 groups; f, late anaphase; g, two daughter nuclei; h, i, polar view of the 

 equatorial plate in the last division; j, anaphase, the gamete nucleus is 

 now haploid (4). a-c, X1840; d-g, X1400; h-j, X3000. 



the nuclei of two gametocytes which encyst together, multiply by 

 mitosis in which eight chromosomes are constantly present (a-g), 

 but in the last division in gamete formation, each daughter nucleus 

 receives only 4 chromosomes (h-j). In another species of Monocystis, 

 Calkins and Bowling (1926) observed that the diploid number of 

 chromosomes was 10 and that haploid condition is established in the 

 last gametic division thus confirming Mulsow's finding. 



In the paedogamy of Actinophrys sol (Fig. 90), Belaf (1923) finds 

 44 chromosomes in the first nuclear division, but after two meiotic 

 divisions, the remaining functional nucleus contains only 22 chromo- 

 somes so that when paedogamy is completed the diploid number is 

 restored. In Polytoma uvella, Moewus finds each of the two gametes 

 is haploid (8 chromosomes) and the zygotes are diploid. The syn- 

 karyon divides twice, and during the first division reduction division 

 takes place. 



