SEX IN PROTOZOA 213 



cronuclcus (10). Two cell divisions, each accompanied by a division 

 of rhc iiiicromiclcus, scr\c to segregate the four niacronuclear anlagen 

 of each exconjuganr into four descendants (11, 12). During these 

 changes the fragments of the old macronucleus gradually become 

 absorbed while the anlagen attain full size. At the end of these two 

 divisions, there are four new cells derived from each conjugant. Each 

 of these cells has a new macronucleus and niicronucleus derived from 

 the svncaryon (12), 



Beginning with Biitschli (1876) and Maupas (1889), students of 

 conjugation have noted deviations from the typical series of stages. 

 Diller (1940b) has called attention to a number of the variations for 

 P. caiidatiim. One nucleus may degenerate at the end of the first pre- 

 gametic division, as happens regularly in P. biirsaria. Instead of three 

 products of the second pregametic division degenerating, tw o, three, 

 or four may begin the third pregametic division, although only one 

 normally completes it to form pronuclei. Among the products of the 

 syncaryon there may be various numbers of nuclei, up to sixteen, 

 with varying fates for the supernumerary nuclei. When the usual 

 eight products are produced, as a rule only four become macronuclear 

 anlagen, but as many as seven may do so. More recently Diller 

 (1950a) has reported an extra postzygotic division in the exconjugant 

 of F. cmidanmi. After the first division one of the daughter nuclei 

 degenerates, the other proceeding to the second division. Two more 

 divisions are necessary to produce the normal eight products derived 

 from the syncaryon. 



Reconjugation between normal individuals and exconjugants has 

 been noted by many observers; the subject is reviewed by Diller 

 (1942), who showed many examples for P. caudatuvi. Figure AA 

 (2 to 4) shows three such reconjugating pairs. As pointed out by 

 Diller, other observers have reported this phenomenon, for example 

 Biitschh (1876) for P. piitmiimi (?), Doflein (1907) and Klitzke 

 (1914) for P. caiidatimt, Miiller (1932) for P. nmltimicronucleatum, 

 and Sonneborn (1936) for P. aurelia. This process has also been de- 

 scribed for other ciliates, for example by Enriques (1908) for 



Fig. AA. Reconjugation in P. caudatum, after Diller (1942), redrawn. 



1, normal exconjugant with four anlagen and four smaller nuclei, three of which 

 are degenerating; 2, 3, exconjugants united with normal animals, all showing crescent 

 stages indicating recent association; 4, normal with two dividing micronuclei united 

 with exconjugant having eight small dividing nuclei, four anlagen present. 



