166 PROTOZOOLOGY 



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. 

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

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

 counting, but Sonneborn's work on the mating types in Parame- 

 cium as quoted elsewhere (p. 186), indicates clearly the occurrence 

 of meiosis during conjugation. 



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

 etes is even more scanty and fragmentary. In Monocystis rostrata, 

 a parasite of the earthworm, Mulsow (1911) 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 Bowl- 

 ing (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, Belaf finds 44 chromosomes 

 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 restored. In 

 Polytoma uvella, Moewus finds each of the two gametes is haploid 

 (8 chromosomes) and the zygotes are diploid. The synkaryon divides 

 twice, and during the first division reduction division takes place. 



In the coccidian Aggregata eherthi (Fig, 217), according to Dobell 

 and Jameson, Belar, 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 reduc- 

 tion takes place also in Gregarina hlattarum (6 to 3, after Sprague, 

 1941) and in Adelina deronis (20 to 10, after Hauschka, 1943). Thus 

 in these forms, the zygote is the only stage in which diploid nucleus 

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

 are haploid. 



Some sixty years ago Weismann pointed out that a protozoan 

 grows and muliplies by binary fission or budding into two equal or 

 unequal individuals without loss of any protoplasmic part and these 

 in turn grow and divide, and that thus in Protozoa there is neither 

 senescence nor natural death which occur invariably in Metazoa in 



