338 SEX IN MICROORGANISMS 



(4) finally, nuclear fusion or karyogamy (such nuclear fusion would 

 eventuaUy become a necessity) . 



Apparently Rhumbler believed that ordinary individuals are dip- 

 loid as they are in Actinophrys sol. Possibly they are haploid. It 

 might be pointed out that cytoplasmic fusion could be followed by 

 nuclear fusion producing diploidy. Haploidy could be restored by a 

 single segregating nuclear division of the type described by Cleveland. 



Since Rhumbler's paper was published, several observers have 

 reported complete fusion in various shelled rhizopods. In 1910 Dan- 

 geard denied the occurrence of karyogamy in Arc ell a, as claimed by 

 Hertwig and his followers (see Swarczewsky, 1908), but later (1937) 

 he stated that "conjugation" including karyogamy took place in 

 Diffltigia globnJosa. Ivanic (1935) described complete fusion includ- 

 ing karyogamy in Cochliopodimn digit atimi, and Penard (1938) re- 

 ported complete fusion for Eiiglypha sciitigera. Other reports are 

 mentioned in the section on Testacea in the paper "Sex in Protozoa." 



CLEVELAND'S IDEAS ON THE ORIGIN OF SEX 



In his discussion of the sexual cycle of the hypermastigote flagel- 

 late, Urinympha, Cleveland (1951) indicates some of his ideas about 

 the evolution of sex. In Uriiiyvipba, which is diploid, the sexual cycle 

 consists of meiosis in one division producing haploid nuclei and the 

 fusion of these nuclei in a process of autogamy. No cytoplasmic 

 divisions or fusions occur, although the "gametocyte" complement 

 of extranuclear organelles is lost as is also one centriole. New extra- 

 nuclear organelles are produced by the centriole which remains. 



Referring to the situation in Barhulanyinpha, which is haploid, 

 variations in its sexual cycle include an endomitotic duplication of 

 chromosomes producing diploidy. This condition persists for about 

 10 days, then meiosis restores haploidy. Barbiilanyinpba also has two 

 other cycles. In one, two gametic nuclei are produced by the mitotic 

 division of the haploid nucleus. These nuclei fuse autogamously with- 

 out cytoplasmic division, then meiosis follows. The other cycle is 

 similar, but cytoplasmic division accompanies the gametogenic mito- 

 sis, producing separate gametes which can fuse in a random manner. 

 Nuclear fusion and meiosis follow. 



As indicated l)y Cleveland, this group of sexual cycles suggests 

 an evolutionary series with the following steps: (I) meiosis to relieve 



