T. M. SONNEBORN 281 



monympha, show complete two-division meiosis, Cleveland de- 

 scribes remarkable abbreviations of, and variations on, the 

 standard sexual processes. According to him, Oxymonas, Saccino- 

 baculus, Notlia, Leptospiwnympha, and Urinympha undergo a 

 remarkable one-division meiosis. Neither centromeres nor chro- 

 mosomes duplicate, chiasmata are not observed, and homologous 

 chromosomes pass intact to opposite poles. This would clearly 

 reduce the possibilities for genetic recombination. Cleveland re- 

 ports for other Flagellates processes which would place even 

 greater restrictions on genetic variability. In Rhynconympha, 

 the first meiotic division is accompanied by cell division, but the 

 second is not, and the sister nuclei thus produced reunite in au- 

 togamy. In Urinympha he maintains that the diploid nucleus 

 undergoes meiosis in one division and that the two reduced nuclei 

 reunite in autogamy. In the haploid Barbulanympha, fusion of 

 sister nuclei is said also to occur sometimes. Some of these 

 Flagellates thus appear to have no possibility of outbreeding. 

 Organisms reach the ultimate limit of inbreeding when fertiliza- 

 tion always takes the form of autogamy. These are the considera- 

 tions concerning host and symbiont variability which led me to 

 conclude that present conditions are not adequate to explain 

 the coincidence in time between infection and fertilization, and 

 that earlier conditions of greater host and symbiont variability 

 probably provided the selective pressure necessary for its evolu- 

 tion. 



A corollary of this conclusion is that the aberrant and abbre- 

 viated sexual processes now observed in the symbionts represent 

 stages in the atrophy and loss of meiosis and fertilization. Cleve- 

 land (1951b) on the contrary rejected this possibility and con- 

 cluded that they were stages in the origin and evolution of 

 sexuality. To him this seemed the more reasonable interpretation 

 of what he recognized as the comparative genetic uselessness 

 of the cytological processes he described. The major support for 

 his view comes from the juxtaposition of two facts : ( 1 ) these 

 symbionts are among the morphologically most complex and 

 highly evolved Flagellates; and (2) sexual processes are not 

 satisfactorily established as occurring in the simpler animal 



