158 SEX IN MICROORGANISMS 



of contact between homologous chromosomes, while the non-homo- 

 logues join to form into two rings which pass intact to opposite poles 

 of the spindle (4, 5 ) . 



During mitosis, after separation of the daughter groups of chro- 

 mosomes, in late anaphase, the middle portion of the elongated nu- 

 clear membrane dissolves and releases the contained granules into the 

 cytoplasm. Cleveland states his belief that such a partial breakdown 

 of the nuclear membrane may be much more prevalent in Protozoa 

 than previous statements would indicate. 



The first meiotic division is like an ordinary cell division in that 

 the parental group of extranuclear organelles is separated into its 

 constituent halves, one of which passes to each daughter cell. A new 

 half set or group develops from each daughter centriole as the parent 

 centrioles function in nuclear division. After the second meiotic 

 division, the half set of extra nuclear organelles associated with the 

 male pronucleus degenerates, leaving the other set to function in 

 the usual manner (6). About 4 days after fusion of the pronuclei 

 the older complement of organelles gradually degenerates while an 

 entirely new group develops from the two centrioles which have re- 

 mained in contact with the nucleus. This is the process of reorganiza- 

 tion, mentioned above; it involves neither nuclear nor cytoplasmic 

 division. 



There are a good many variations from the series of events de- 

 scribed above. For example, instead of pseudoencystation, sometimes 

 true cysts are formed (10) in which there is no development. 



Certain of the more important aspects of the sexual cycles of 

 these flagellates living in the gut of Cryptocercus are shown in Table 

 II. There it will be seen that, of the three polymastigote flagellates, 

 Oxymonas and Saccinobaciilus are haploid and that postzygotic meio- 

 sis is accomplished in one division. Notila is diploid, but again the 

 pregamic meiosis involves only one division. 



In the hypermastigote group, Leptospironympha, Trichonym- 

 pha, and Eiicomonympha are haploid. The postzygotic meiosis of 

 Leptospironympha is accomplished by a single division, whereas in 

 Trichonyrnpha and Eucomonyjnpha there are two meiotic divisions. 

 The diploid Urinympha achieves pregametic meiosis in a single divi- 

 sion and autogamy takes place, whereas the other diploid, Rhyjicho- 

 iiy/npha, requires two pregametic meiotic divisions to produce haploid 

 gametes which fuse in autogamy. Not only are these diversities in 



