616 FERTILIZATION 



drawn from the many diverse observations and interpretations of the 

 conditions under which fertihzation occurs in ciliates, viz., the proto- 

 plasmic state with which conjugation is possible is induced in large part, 

 but not wholly, by environmental conditions." 



It is a matter of common observation that when conjugation occurs 

 in mass cultures, all the ciliates do not conjugate, but only a certain 

 proportion of them. The proportion may vary with the culture and the 

 species, but in any case if the conditions in the mass culture are favorable 

 for inducing conjugation in some individuals, why do they not all con- 

 jugate? The fact that some do and some do not conjugate under condi- 

 tions that appear to be identical, would indicate the existence of internal 

 differences. 



Calkins (1933, p. 290) summarizes the evidence and concludes "that 

 environmental stimuli are without effect in producing conjugations un- 

 less the protoplasm is in a condition where such conjugations are pos- 

 sible." Two examples illustrate different phases of this proposition: 

 Uroleptus moh'tlh will conjugate only after a period of from five to 

 ten days after fertilization, and stock R of Paramecium aurelia (Sonne- 

 born, 1936) will conjugate only in descendants of animals which have 

 recently undergone conjugation or endomixis. The time factor is ob- 

 viously different in these two animals, but both clearly indicate a strong 

 cyclical differentiation which affects conjugation. 



For more detailed analyses of this subject, reference should be made 

 to Calkins's Biology of the Protozoa (1933) and to Chapter XIV below, 

 by Sonneborn. 



There is, perhaps, even less agreement concerning the effects of con- 

 jugation than concerning the causes. In some ciliates, e.g., Uroleptus 

 mobilis (Calkins, 1919), conjugation results in a definite renewal of 

 vitality, as indicated by an increase in the fission rate. Calkins interprets 

 this as a fundamental process, which is an integral and normal part of 

 the life cyle. Woodruff and Spencer (1924) found a similar renewal 

 of vitality following conjugation in Spathidium spathula, but Woodruff 

 (1925) interprets this as a rescue process to "meet the emergency of 

 physiological degeneration induced by environmental conditions which 

 are not ideal." Beers (1931) shows that conjugation increases vitality in 

 Didinium nasutum which has been depressed by inadequate feeding, but 

 that no depression occurs in well-fed animals. 



