SEX IN PROTOZOA 245 



autogamy also occurs as a variation of conjugation, as previously 

 shown for various species of Faramecium. Furthermore, autogamy is 

 widely distributed among the Protozoa, seemingly being character- 

 istic for the Cnidosporidia and Haplosporidia, and occurring in Helio- 

 zoa, and possibly in the Radiolaria, and various animal flagellates 

 (Cleveland's studies). If ciliates have evolved from flagellates where 

 autogamy as well as union of gametes of diverse parentage occurs, 

 the appearance of autogamy in ciliates should not be surprising. Un- 

 fortunately, Calkins and Bowling found that the zygotes resulting 

 from autogamy were mostly non-viable. This might suggest that 

 conjugation, providing for cross fertilization, would have a greater 

 survival value than autogamy. One could not say the same for the 

 Cnidosporidia and Haplosporidia, in which autogamy is the rule. 



In the preceding consideration of sexual phenomena in ciliated 

 Protozoa, no attempt has been made to describe the occurrence and 

 behavior of mating types, since discussions and interpretations of 

 mating-type phenomena are provided by Dr. Nanney and Dr. Metz 

 in the next two papers. 



SUMMARY 



Among the Phytomastigina, syngamy is common in the Phy- 

 tomonadina (see paper by Lewin). It is uncommon in the Chryso- 

 monadina, Euglenoidina and Dinoflagellata, and in these groups no 

 haploid-diploid cycles have been fully demonstrated except for the 

 chrysomonad, Ochrosphaera JieopoHtana. 



In the Zoomastigina, cell fusions have been described for scat- 

 tered representatives of the Rhizomastigina, Protomonadina and Poly- 

 mastigina, but without adequate cytological details. According to 

 Cleveland, however, the flagellates that inhabit the gut of the wood- 

 feeding roach, Cryptocerciis pimctiilatiis, exhibit sexuality whenever 

 the host molts, and detailed cytological descriptions have been pro- 

 vided for certain species. Haploid flagellates undergo postzygotic 

 meiosis, some kinds with one, some with two divisions. Diploid flagel- 

 lates show pregametic meiosis which requires two divisions in some 

 species, and only one in others. Autogamy occurs in some species. 

 In Trichojiyinpha and some other haploid flagellates, each gameto- 

 cyte produces a male and a female gamete by a sex-differentiating 

 mitosis during which each parental chromosome gives rise to dif- 



