THE PHYSIOI.OCIV OK IKRTILIZA 1 ION IN CILIATFS 30I 



Nevertheless, this third animal undergoes a complete nuclear cycle 

 (cvtogamv). 



Holdfast union, the second union in conjugation, is also not an 

 essential preliminary to activation in Paraviechmi. This is evident 

 from the fact that the reaction between dead and living animals, 

 which leads to activation of the latter, does not involve a holdfast 

 attachment. Although this evidence clearly eliminates typical hold- 

 fast union as a possibility, it cannot be considered a critical elimina- 

 tion of holdfast substances from participation in the activating reac- 

 tion. Since the dead animals are derived from normal cultures whose 

 members can form holdfast unions, it is possible that they possess 

 special preformed holdfast substances. These might interact with 

 similar substances on the living animals even though this interaction 

 does not lead to lasting union. The same argument may be applied to 

 paroral substances. 



Both holdfast and paroral substances were clearly eliminated as 

 essential factors in the initiation of activation by a study of a mutant 

 stock of F. aurelia (Metz and Foley, 1949). Animals of this CM 

 ("can't mate") stock give good mating reaction under appropriate 

 physiological conditions. However, these animals are incapable of 

 proceeding further in the conjugation process. Thus CM animals 

 never form holdfast or paroral unions with CM or normal animals. 

 Furthermore the CM animals do not undergo loss of mating activity, 

 meiosis, or macronuclear breakdown through sexual association with 

 CM or normal animals. In short, CM animals cannot be activated by 

 CM or normal animals of opposite mating type. Nevertheless, CM ani- 

 mals can induce loss of mating activity, holdfast union ("pseudo 



TABLE IV 



Activating Properties of CM and Normal Animals of Opposite Type 



(P. aurelia, Var. 4) 



