C. DOBBLL 165 



" homogamy " in Paramecium means much. It is merely an expression 

 of the fact that a population of sexually mature individuals is more 

 uniform in size than is a population of adults, adolescents, and 

 children. 



92. Is there a "eugamic period," or period of "karyogamic maturity," 

 in the life-cycle of a ciliate ? Many experiments could be quoted to 

 show that there is not. For instance. Calkins (1902) concludes: 

 " ' Karyogamic maturitj' ' does not signify much when fertile unions 

 occur with individuals' in the 350th, the 410th, the 4.67th, and .500th 

 generations of the same culture." And Jennings (1913) records an 

 experiment (No. 9) in which he studied two lines of P. awelia, both 

 derived from one original ancestor, but only one of which had been 

 allowed to conjugate. "Under the proper conditions both sets con- 

 jugate at the same time, in spite of the fact that one has conjugated 

 at least four times since the other." These and similar observations 

 (especially those on reconj ugation — § 120 infra) seem to indicate clearly 

 that a " eugamic period," as conceived by Maupas, does not exist°. 



93. R. Hertwig (190.5) states that when Dileptus is starved^ two 

 rapidly succeeding divisions ("hunger divisions") take place, resulting 

 in the formation of four small individuals from each original individual. 

 This is said to occur also in Didinium (Prandtl, 1906) and Paramecium 

 (Kasanzetf, 1901). "Since Infusoria which have undergone hunger- 

 divisions proceed to conjugate, a causal connexion seems to exist 

 between the two phenomena. The hunger-divisions correspond in 

 this respect with the maturation divisions of multicellular organisms." 

 It is true that these " hunger-divisions " may account for the smaller 

 size of conjugants — when they are smaller: but the remainder of 

 Hertwig's argument seems based on a false analogy. For since the 

 conjugant is not a gamete but a sexual individual (§ 12), it cannot 

 properly be compared with a germ-cell. The phenomena in a ciliate . 

 which are homologous with the two meiotic divisions in a metazoon 

 are the micronuclear divisions preceding fertilization (§ 13). 



' Paramecium caudatum. 



" It is possible, however, that under normal conditions conjugation maj' occur with 

 fairly regular periodic frequency. Jennings (1910) describes a race of Paraimeium which, 

 under suitable conditions, will conjugate again 5 days after conjugation: in other races 

 there was an interval of "a year or more." Something like a "eugamic period "may 

 couceivably exist, therefore, though it seems very improbable that this is the "explanation" 

 of these facts. 



' Hunger, it will be recalled, is a condition conducive to conjugation, according to 

 Maupas (§ 20). 



