226 ] The Classification of Lower Organisms 



so that when mitosis is complete each protoplast contains two haploid nuclei of dif- 

 ferent origin. In other ciliates the same result is attained, apparently, by the migration 

 of one nucleus of each pair. Karyogamy takes place in each conjugant. The cyto- 

 plasmic connection is broken and the conjugants separate from each other. During 

 several subsequent hours, the zygote nucleus undergoes a characteristic number of 

 divisions, three in Paramaecium. Among the nuclei produced, one usually enlarges 

 and becomes a macronucleus; others, of the number characteristic of the form, survive 

 as micronuclei; the remainder are digested. 



In Vorticella and its allies, syngamy consists of the complete fusion of a smaller 

 swimming individual with a larger one attached by a stalk. The nuclear processes 

 are believed to be essentially as in other ciliates. The reproduction of the Tentaculi- 

 fera has not been much studied, but here also the nuclear changes are as in the 

 generahty of ciliates (Noble, 1932). 



The possibility of conjugation is limited by the occurrence of mating types. Certain 

 early observations had suggested the existence of these; the definite discovery was by 

 Sonneborn, in Paramaecium Aurelia (1937). Results of further study are available 

 in a symposium edited by Jennings (1940) and in a review by Kimball (1943). To 

 current knowledge, then: 



Paramaecium caudatum includes four mating types divided into two groups; types 

 I and II conjugate with each other, and types III and IV with each other, but the 

 two groups are mutually sterile. 



Paramaecium Aurelia includes eight mutually sterile groups, each of two mutually 

 fertile mating types. 



Paraviaecium Bursaria includes three mutually sterile groups. The first group is 

 of four types, each self-sterile but able to conjugate with any other; the second group 

 is of eight such types, and the third again of four. 



Paramaecium. multimicronucleatum is without mating types; any race can conju- 

 gate with any other 



Euplotes Patella includes six mating types all in one group; each can conjugate 

 with any other. 



The heredity of mating types is not understood. It is not a matter of simple Men- 

 dclian heredity. In Paramaecium Bursaria group I, the progeny of a cell of a given 

 mating type may include after conjugation either two or all four of the mating types. 

 The mating type of a line becomes fixed in connection with the first or second cell 

 division after conjugation, at the time that macronuclei are being differentiated; it 

 is accordingly believed that something in the macronuclei fixes the mating types. 



So far as mating types are present, pure lines of ciliates cannot conjugate. Early 

 attempts to maintain pure cultures failed by death after intervals of some months. 

 These observations led to speculations that the vitality of protoplasm is limited, and 

 that sexual reproduction restores it. Woodruff, however, proved it possible to maintain 

 Parflmagau?n ^urc/m indefinitely without conjugation: he reported (1926) a culture 

 so maintained for sixteen years, an estimated eleven thousand generations. 



The cultures are not thus persistent without nuclear change. At intervals, the macro- 

 nuclei break up and dissolve, and are replaced by new ones formed by division of the 

 micronuclei. Woodruff and Erdmann (1914) applied to this process of replacement 

 of nuclei the term cndomixis. It is not possible that this process is the genetic equiva- 

 lent of karyogamy. It is, presumably, the physiological equivalent of conjugation in 

 its feature of providing new macronuclei. 



