214 SEX IN MICROORGANISMS 



Chilodonella uncinatus, and by Collin (1912) for various suctorians. 

 Wichterman (1939, 1940) placed conjugating pairs of P. cauda- 

 tmn under a precision microcompressor which enabled him to observe 

 nuclear phenomena in living animals. Under these conditions he dis- 

 covered that in each conjugant self-fertilization (autogamy) took 

 place instead of cross fertilization. He called this process "cytogamy." 

 Cross fertilization probably occurs in most instances of conjugating 

 pairs, as shown by Diller (1950b). 



Conjugation in Other Species of Parajneciujn 



Different species of Paramecium show varying degrees of diver- 

 gence from the account just given for P. caudamm. P. biirsaria and 

 P. trichhim are like P. caudatnm in that each has a single, rather large 

 micronucleus, whereas P. aiirelia and P. calk'msi usually have two 

 smaller, more vesicular micronuclei, and P. imiltimicronucleatum, 

 P. njooodnifji, and P. poly car ywn commonly have four micronuclei of 

 the P. ajirelia type. 



Although P. biirsaria has a micronucleus much like that of P. 

 caudatwn, the stages of conjugation are somewhat different. For 

 accounts of conjugation in this species we may refer to those of 

 Maupas (1889), Hamburger (1904), Chen (1940a, b, c, d; 1946a, b; 

 1951a, b) and Wichterman (1948a, 1953). According to these authors 

 each of the first two pregametic divisions are followed by degenera- 

 tion of one of the products. The remaining haploid nucleus divides 

 to provide the two pronuclei. According to Maupas and Hamburger, 

 after exchange of pronuclei and syncaryon formation, two divisions 

 provide four nuclei, two of which become macronuclei and two 

 micronuclei. One cell division segregates these into two animals each 

 with one nucleus of each kind. However, Chen (1946b, 1951a,b) 

 and Wichterman (1948a) found that after the first postzygotic nu- 

 clear division one nucleus degenerates, so that it takes two more 

 divisions to produce the four products which develop into two macro- 

 and two micronuclei. In this species the old macronucleus does not 

 fragment but undergoes gradual absorption in the exconjugant. 



All these authors have recorded variations in the conjugation of 

 this species. Hamburger (1904) recorded passage of a pronucleus 

 from one conjugant to the other without receiving a pronucleus 

 from the partner, leading to one "haploid" and one "triploid" indi- 



