FERTILIZATION 615 



the different schools of thought regarding the conditions necessary for 

 conjugation in ciliates, upon which most of this work has been done. 



In regard to ancestry, Calkins (1904) found that in Paramecium 

 caudatum there are fully as many conjugations between closely related 

 individuals as between individuals of diverse ancestry. He further indi- 

 cates (Calkins 1933) that similar results have been obtained through 

 isolation cultures of Didinhim nasutum, P. aurelia, P. bursaria, Styl- 

 nychia sp., Blepharisma tindulans, Spathidium spathula, Oxytncba jal- 

 lax, Chilodonella cucidlus, and Uvoleptus mohilis. Sonneborn and Cohen 

 (1936) found that under identical conditions, "The Johns Hopkins 

 stock R of Paramecium aurelia can invariably be induced to conjugate," 

 while "the Yale stock of the same species cannot." This difference ap- 

 pears to be clearly racial. Sonneborn's (1937) discovery of two "sex 

 reaction types" in a race of P. aurelia may throw considerable new light 

 on this question. Members of one type readily conjugate with those of 

 the other type, but do not conjugate among themselves. 



At first this looked as though something resembling the two sexes of 

 other organisms had been found in the reaction of one ciliate to another. 

 However, the discovery by Jennings (1938) of as many as nine sex 

 reaction types in P. bursaria seems to remove these types from the cate- 

 gory of sexes and indicates that they are simply strains which will not 

 inbreed. The significance of these discoveries is not yet clear, but they 

 do show that in some cases, at least, diverse ancestry is a potent factor 

 in conjugation. 



In regard to the relative importance of external conditions and in- 

 ternal conditions in ciliate conjugation, we again find contradictory evi- 

 dence if we generalize from specific instances. The inductive method of 

 reasoning is certainly stimulating and productive, but its misuse has led 

 to some unjustifiably broad propositions. There is a rapidly accumulat- 

 ing array of evidence that external conditions, such as food, tempera- 

 ture, pH, population concentration, light, seasons, chemicals, condition 

 of host in some parasitic forms, and so forth, do play an important role 

 in inducing conjugation in some ciliates. However, there is valid evi- 

 dence to indicate that in some forms, at least, conjugation can be in- 

 duced only at certain times in the life cycle of the organism — in other 

 words, only when the protoplasm is sexually "mature" for conjugation. 

 Calkins (1933, p. 286) states that "One unmistakable conclusion can be 



