REPRODUCTION 159 



in number of chromosomes in the Protozoa. In conjugating 

 Didinium nasutum, 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 poly- 

 piniim (Popoff), Uroleptiis halseyi (Calkins), etc. (see the ciliates 

 in the list on p. 135). In various species of Paramecium and many- 

 other forms, the number of chromosomes appears to be too great 

 to allow a precise counting, but it is generally agreed that here 

 probably reduction in the number also takes place. 



Information on the meiosis involved in the complete fusion of 

 gametes is even more scanty and fragmentary. In Monocystis 

 rostrata, a parasite of the earthworm, Mulsow, noticed that the 

 nuclei of two gametocytes which encyst together, multiply by 

 mitosis in which eight chromosomes are constantly present, but 

 in the last division in gamete formation, each daughter nucleus 

 receives only 4 chromosomes. 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 Actinopkrys sol, Belaf found 44 chromo- 

 somes in the first nuclear division, but after two meiotic divisions, 

 the remaining functional nucleus contains only 22 chromosomes 

 so that when paedogamy is completed the diploid number is re- 

 stored. On the other hand, in the coccidian Aggregata eberthi 

 (Fig. 190), according to Dobell and Jameson, Belaf, and Naville, 

 and in the gregarine Diplocystis schneideri, according to Jameson, 

 there is no reduction in the number of chromosomes during the 

 gamete-formation, but the first zygotic division is meiotic, 12 to 6 

 and 6 to 3, respectively. A similar reduction in chromosome (12 

 to 6) takes place also in the gregarine Zygosoma glohosum, accord- 

 ing to Noble's recent study (1938). Thus it appears in these cases 

 that the zygote or oocyst is the only stage in which diploid nucleus 

 occurs, while the nuclei in the stages in the remainder of the life- 

 cycle are haploid. 



References 



Belar, K. 1926 Der Formwechsel der Protistenkerne. Ergebn. 

 u. Fortschr. Zool., Vol. 6. 



