COMMENTS ON THE ORIGIN AND EVOLUTION Ol SI X 339 



cndoniirotic diploidv; (2) iiiciosis followed by autogamy, but with- 

 out cell division, thus without gamete formation; (3) gametogencsis 

 in haploid cells followed by fertilization, then meiosis, restoring 

 haploidv; (4) establishment of diploidy, followed by simultaneous 

 gametogenesis and meiosis, then fertilization, restoring diploidy; (5) 

 differentiation of the "sexes" as to gonads and secondary sexual char- 

 acters. 



Cleveland states that, before the appearance of meiosis, unicellu- 

 lar organisms were perhaps haploid. Diploidy could arise by endo- 

 mitosis in a haploid nucleus without compensating meiosis. When 

 asexual cells are haploid, meiosis and gametogenesis cannot occur 

 simultaneously; meiosis is zygotic. When asexual cells are diploid, 

 gametogenesis and meiosis must occur concomitantly. 



In sexual cycles, as pointed out by Cleveland, even when no 

 gametes are produced, a centriole is lost, as occurs in higher types of 

 cycle, since one gamete loses its centriole somewhere during the 

 process of gamete formation and fusion. Hence the loss of a cen- 

 triole, like meiosis, antedated gametogenesis in sexual evolution. 

 Therefore a study of the origin of sex should begin with a considera- 

 tion of environmental agents responsible for meiosis, that is, those 

 that induce suppression of duplication of centromeres and chromo- 

 somes. Of secondary importance are the agencies responsible for dif- 

 ferentiation into male and female gametes, their union, and the for- 

 mation of male and female characters. 



EVOLUTION OF "SEX" 



W^ith these theories of the origin of syngamy before us, we may 

 pursue the subject of the evolution of this process further. 



An examination of the status of sexuality in the Protozoa reveals 

 that the groups in which syngamy is most commonly found are those 

 which are more highly evolved. Among the Sarcodina these are the 

 Foraminifera, Heliozoa, and possibly the Radiolaria, with only scat- 

 tered examples, often unconfirmed, in the Proteomyxa, Amoebaea, 

 and Testacea. Among the Sporozoa syngamy seems to be general in 

 the Gregarinida, Coccidia, and Haemosporidia; also common or gen- 

 eral in the Myxosporidia, Actinomyxidia, some Aiicrosporidia and 

 Haplosporidia. In the Ciliophora, syngamy of a relatively primitive 

 type is described for the Protociliata, w^hereas in the Euciliata and 



