190 PROTOZOOLOGY 



in the quantity of chromatin it contains, which gives rise to dif- 

 ferent (about 80 to several hundred) chromosome numbers during 

 conjugation in different races, and that polyploidy is not uncom- 

 mon in this ciliate. This investigator considers that polyploidy is 

 a result of fusion of more than two pronuclei which he observed on 

 several occasions. The increased number of pronuclei in a conju- 

 gant may be due to: (1) the failure of one of the two nuclei produced 

 by the first or second division to degenerate; (2) the conjugation 

 between a unimicronucleate and a bimicronucleate, or (3) the fail- 

 ure of the wandering pronucleus to enter the other conjugant; with 

 this latter view Wichterman (1946) agrees. Apparently polyploidy 

 occurs in other species also; for example, in P. caudatum (Calkins 

 and Cull, 1907; Penn, 1937). 



In P. trichium, Diller (1948) reported that the usual process of 

 conjugation is the sequence of three micronuclear divisions, pro- 

 ducing the pronuclei (during which degeneration of nuclei may oc- 

 cur at the end of both the first and second divisions), cross- or 

 self-fertilization and three divisions of the synkarya. Ordinarily four 

 of the eight nuclei become macronuclei, one remains as the micro- 

 nucleus and the other three degenerate. The micronucleus divides 

 at each of the two cytoplasmic divisions. Exchange of strands of the 

 macronuclear skein may take place between the conjugants. Diller 

 found a number of variations such as omission of the third prefer- 

 tilization division, autogamous development, etc., and remarked 

 that heteroploidy is pronounced and common. 



In P. aurelia possessing typically two micronuclei, the process of 

 conjugation was studied by Maupas (1889), Hertwig (1889), Dil- 

 ler (1936), Sonneborn (1947), etc., and is as follows: Soon after bi- 

 association begins, the two micronuclei in each conjugant divide 

 twice and produce eight nuclei, seven of which degenerate, while the 

 remaining one divides into two gametic nuclei (Maupas, Woodruff, 

 Sonneborn) Diller notes that two or more of the eight nuclei divide 

 for the third time, but all but two degenerate; the two gametic nu- 

 clei may or may not be sister nuclei. All agree that there are two 

 functional pronuclei in each conjugant. As in other species of Para- 

 mecium already noted, there is a nuclear exchange which results in 

 the formation of a synkaryon in each conjugant. The synkaryon di- 

 vides twice and the conjugants separate from each other at about 

 this time. Two nuclei develop into macronuclei and the other two 

 into micronuclei. Prior to the first cytoplasmic division of the excon- 

 jugant, the micronuclei divide once, but the macronucleus does not 

 divide, so that each of the two daughters receives one macronucleus 



