SEX IN PROTOZOA 219 



autogiiniy (colinn d) may occur. Ihc liaploid nuclei produced by the 

 two prcgametic divisions may fail to form pronuclei, but pass to their 

 mates (column e) or develop parthenogenetically (column f.) Some 

 of these variations arc illustrated in Fig. AD. Transfer of a single 

 nucleus into a mate with degenerating nuclei is shown at (1), leaving 

 one conjugant without a nucleus. At (2) a binucleate specimen is 

 conjugating with a uninucleate one. In (3) and (4) exchange of 

 macronuclear material is taking place. In (3) one pronucleus has 

 transferred without reciprocal exchange, producing a "haploid" in- 

 dividual at the left and a "triploid" at the right, as previously men- 

 tioned for P. bur sarin. Diller also saw conjugation between a normal 

 animal and the anterior member of a tandem pair, as described above 

 for P. bursaria. Later Diller (1949) described an abbreviated process 

 of conjugation in P. trichm-ni, where exchange of nuclei took place 

 after the first pregametic division. The products of the first division 

 proceed directly to reconstitute the new nuclear apparatus by syn- 

 karyon formation or parthenogenesis, or a combination of the two. 

 There is no degeneration of nuclei between divisions. 



The account of conjugation in P. piitrijiwn by Doflein and 

 Reichenow^ (1949) follows closely the "standard" series of events in 

 P. trich'mm, as shown in Fig. AC, column a. As stated by Diller, it 

 is possible that Doflein's account refers to P. trichmm. The identity 

 of P. piitrinuvi is still in doubt (Wenrich, 1926). 



For Paramecium aiirelia, with two small micronuclei, accounts 

 of conjugation have been supplied by a number of authors, especially 

 by Hertwig (1889), Maupas (1889), Diller (1936), and Sonneborn 

 (1947). There is general agreement that the two micronuclei go 

 through a crescent stage previous to the first pregametic division. 

 The four nuclei produced by this division quickly go through the 

 second division to produce eight haploid nuclei. According to Hert- 

 wig and Sonneborn seven of these nuclei degenerate while the remain- 

 ing one divides to produce the pronuclei. However, Diller stated that 

 a variable number, from two to five, of these eight nuclei at least 

 begin the third division, but only two of the resulting nuclei, appar- 

 ently those nearest the paroral cone, become pronuclei. As suggested 

 by Diller (1936) and Sonneborn (1951), the position in the cell 

 apparently determines which haploid nuclei become functional. Son- 

 neborn (1951) cites this as an example of cytoplasmic control over 

 nuclear activities. After exchange of pronuclei and syncaryon forma- 

 tion, the two conjugants separate and two mitotic divisions produce 



