SEX IN PROIOZOA I57 



Between molts of the host the diploid Notila (Fig. H, 1) (Cleve- 

 huul, 195()c) divides by mitosis with twenty-eight chromosomes. 

 About 7 d;ivs before molting of the roacli occurs, this flagellate be- 

 comes a gametocNtc without an\- obvious morphological changes. In 

 a single division somewhat more rapid than usual (2), this jrametocvte 

 produces a male and a female diploid "gamete," the latter being some- 

 w hat larger. Each "gamete" possesses four flagella, an axostyle, and 

 two intranuclear centriolcs. These "gametes" increase in size, then 

 fuse, first the cytoplasm (3), then the axostyles (4), but not the 

 nuclei. In this condition the organisms remain for 7 to 8 days. Then 

 the male nucleus becomes detached from its axostyle, moving to 

 another part of the cytoplasm but retaining for a time its four fla- 

 gella and two centrioles. 



Next meiosis occurs. This is nearly synchronous in the two 

 nuclei (5), There is no duplication of chromosomes in connection 

 with this division; hence reduction is accomplished by one division. 

 Two male and two female haploid nuclei, each with fourteen chro- 

 mosomes, are produced. Soon after meiosis is completed, fusion of 

 the pronuclei begins. iV male pronucleus ahvays fuses with a female 

 pronucleus, thus forming two diploid nuclei (7). Cytoplasmic divi- 

 sion of this "double zygote" produces two diploid asexual cells. There 

 are, of course, occasional deviations from the account just given. 



Hypeniiastigotes. The first hypermastigote flagellate for which 

 a sexual cycle was described by Cleveland (1949) was Trichonyin- 

 pha. All the species of this genus go through the same cycle simul- 

 taneously, and all are haploid with twenty-four chromosomes during 

 periods between molts of the host. 



Gametogenesis begins about 3 days before the host molts. At 

 molting time gametocytes are encysted, and nuclear division has taken 

 place but not cytoplasmic division. After molting, development of 



Fig. T. Trichonympha, from Cleveland (1949). 



1, a gametocyte in early stage of encystation; 2, detail of anterior end, duplication 

 of chromosomes has occurred, note difference in staining in nucleus; 3, vertical 

 view of metaphase-anaphase, most of male chromatids going to one pole and most 

 of female chromatids to the other, interconnections between light and dark groups 

 result from homologous pairing; 4, gametocyst of smaller species, gametes have more 

 space in which to move, are about ready to excyst; 5, male gamete is almost 

 halfway in the female gamete and has already become slightly smaller owing to some 

 of its cytoplasm having been passed to female. 



