SEX IN PROTOZOA 167 



point of view . This aspect of the subject is discussed in the paper 

 "Coninicnts on the Origin aiid I'Aokition of Sex." 



In its morphology Rhyiicbonyinpba (Cleveland, 1952) is inter- 

 mediate l)et\\een Urinynipba and Bcirbiihrnympha. Its sexual cycle 

 shows similarities to that of both these genera but has features pecul- 

 iar to itself. 



Sexual activities of diploid Rbyiicbonynipba tarda (Fig. M, 1) 

 begin about 9 days before the host molts and end 10 to 12 hours after 

 molting. First there is a meiotic nuclear division (2, 3) accompanied 

 by cytoplasmic division. In the cells thus produced a second meiotic 

 nuclear division (4, 5) takes place without cell division. The two 

 haploid pronuclei thus formed fuse in a process of autogamy (6 to 9). 

 This series of events takes about 2 days. The resulting cell, which 

 Cleveland hesitates to call a zygote, increases considerably in size for 

 about 2 days, then undergoes a reorganization during which the old 

 set of extranuclear organelles gradually degenerates while a new set 

 develops from two centrioles. Pseudoencystation, which lasts about 3 

 days, follows. During this period the flagellate may be passed along 

 to ne\\'lv hatched and uninfected hosts. After some 12 to 15 hours 

 the flagellates resume their activity, and a period of rapid cell divi- 

 sions ensues. 



In Rbyncbonyvipba the size of the chromosomes {2N = 20) 

 varies considerably, but all have median or submedian centromeres, 

 in contrast to those of Urinympba, which are similar in size and have 

 terminal centromeres. In the first meiotic division, homologous chro- 

 mosomes pair only at their distal ends, so that no tetrads in the ordi- 

 nary sense are produced and crossing-over is thus practically im- 

 possible. Lateral joinings of non-homologous chromosomes produce 

 groupings on each side of the equatorial region, as described for 

 TricboiiyiJipba and Leptospironyvipba (2, 3). In the second meiotic 

 division the behavior of the chromosomes is very similar to that in 

 the first, except that there is no duplication, there being a minimum 



Fig. M. Rhynchonympha tarda, from Cleveland (1952). 



1, entire animal, showing organization; 2, metaphase or early anaphase of first meiotic 

 division, non-homologous chromosomes joined into two rings, homologous chromo- 

 somes connected at the equatorial plane; 3, details of stage shown in 2; 4, metaphase of 

 second meiotic division, two rings of non-homologous chromosomes, as in 2; 5, 

 details of stage shown in 4; 6, stage in fusion of pronuclei, organelles attached to 

 female nucleus; 7-9, successive stages of pronuclear fusion; 10, encysted animal. 



