SEX IN PROTOZOA 155 



the dauQ^hter cells whicli ;irc gaiiicrcs (2). In Sacct7iobaciiliis ambloax- 

 ostylus the parental axosrxle is nor discarded until the mctaphasc of 

 nuclear division, and the parental flagella are retained, two going to 

 each daughter, two new ones being developed from new centrioles 

 arising by division of parental centrioles, a new axostyle growing out 

 from each new centriole. 



In Oxv7no/his, two gametes meet and fuse (3), first the cyto- 

 plasm, then the axostyles (4), and finally the nuclei. Since gameto- 

 genesis begins 6 to 7 days before molting of the host, and meiosis, 

 which is zygotic, does not begin until approximately 1 day after 

 molting, the zygote stage lasts at least 6 days with no loss of extra- 

 nuclear organelles. In Sacciiwbaciihis, after cytoplasmic fusion (5), 

 nuclear fusion is delayed 2 or 3 days during which the cells become 

 spherical and remain rather inactive until the host molts several days 

 later. Within 8 to 12 hours after molting of the roach, the zygote 

 becomes more active, and, some 20 to 24 hours after molting, meiosis 

 (6) begins, to be completed in about 24 hours. 



In the usual two-division meiosis the chromosomes are duplicated 

 during the first division, whereas the centromeres are not. In the 

 second division the centromeres are duplicated but not the chromo- 

 somes. In both these flagellates meiosis is accomplished by a single 

 nuclear division durino- which neither chromosomes nor centromeres 

 are duplicated. Only slight pairing takes place, and the chromosomes 

 move to the poles as monads, as seen in the second meiotic division 

 of most other animals. Random segregation takes place. Cytoplasmic 

 division of the zygote completes the formation of two haploid aga- 

 metes. 



In Oxymonas nana essentially similar phenomena take place, but 

 in this species the zygote encysts. Later, excystation and division into 

 haploid agametes occur. 



Fig. H. Notila proteus, from Cleveland (1950c). 



1, agamont with nucleus, nucleoli, four flagella, axostyle, and axostylar granules; 2, 

 gametocyte shortly before cytoplasmic division, each nucleus with four flagella and 

 a half-grown axostyle; 3, axostyles of fused gametes beginning to fuse at their poste- 

 rior ends; 4, chromosomes of male and female nuclei have regained their major 

 coils preparatory to meiosis, male nucleus has become dissociated from fused 

 axostyles and is moving away; 5, male nucleus has moved to the posterior end of 

 the cell continuing its development, old fused axostyles still connected with female 

 nucleus which has two new axostyles and eight flagella; 6, early anaphase of male 

 nucleus, the female nucleus at this stage presents the same picture; 7, each male nu- 

 cleus is fusing with a female nucleus. 



