SEX IN PROIOZOA ]91 



gregarines. Most cases of prcganicric mciosis have been reported for 

 members of the Acephalina (Table III), while most members of the 

 Cephalina sliow post/.vtjotic meiosis, especially in more recent reports 

 (Table 111). 



For the acephaline gregarines various authors including iVlulsow 

 (1911), Calkins and Bowling (1926), and Naville (1927a, c) have 

 reported pregamctic meiosis, while Jameson (1920) and others de- 

 scribed post/.vgotic mciosis. Figure T (1 to 48) shows selected illu- 

 strations from Naville's (1927a) paper on Urospora lagidis. Naville 

 illustrated mitoses for the last four divisions in gametogenesis. Figure 

 T shows mitoses for both female (1 to 8) and male (9 to 16) gamonts 

 for the second stage where four chromosomes divide and separate. 

 In the third stage (17 to 32), onlv two chromosomes go to each pole 

 in the anaphases. The fourth stage (not illustrated here) also shows 

 two chromosomes going to each pole. Thus there are two meiotic 

 divisions according to Naville. Zygote formation and development 

 of the eight sporozoites within it are also illustrated (33 to 48). Four 

 chromosomes appear in the mitoses. The evidence for postzygotic 

 meiosis offered by Jameson (1920) seems equally clear. Grell (1940) 

 after a critical review of the literature thought that probably all greg- 

 arines had postzygotic meiosis. 



If it is true that some gregarines exhibit pregametic and others 

 postzygotic meiosis, then it may make little difference whether the 

 trophic existence is under the control of a haploid or a diploid set 

 of chromosomes. Since, however, the more highly evolved cephaline 

 gregarines (Table III) and Coccidia (Table IV) generally have post- 

 zygotic meiosis, these groups are apparently under no handicap by 

 living a haploid existence, the zygote being the only diploid stage in 

 the life cycle. Why should the more primitive (?) acephaline greg- 

 arines show both types of cycler If the gregarines and Coccidia 

 have evolved from the flagellates, as has been suggested by a number 

 of authors, for example BiitschU (1882-89), Minchin (1912), Alexei- 

 eff (1912), and Cleveland (1949), one wonders if the diploid and 

 haploid condition in some gregarines represents a carry-over from 

 diploid and haploid flagellated ancestors. In the group of flagellates 

 living in Cryptocerais, both haploid and diploid cycles are found. 

 This may indicate that neither type of cycle has become predominant 

 in the flagellates, although such evidence as we have indicates that 

 free-living flagellates are usually haploid. Since diploidy could arise 



