20 PHYSIOLOGICAL TRIGGERS 



The dead paramecia described above may be regarded as a collection of 

 substances adsorbed to or built into the surface of an inert carrier, the body of 

 the dead animal. Evidently this collection of substances includes the specific 

 mating type substances. Furthermore, it must include the substance or sub- 

 stances that interact to activate conjugating paramecia. The problem of central 

 interest, then, is to identify and characterize these substances and describe 

 the mechanism of their interaction and physiological action. 



Two main lines of evidence indicate that the activating substances are the 

 mating type substances, the substances which are located on the surfaces of 

 the cilia and which, by definition, interact in the initial adhesive reaction, 

 the mating reaction. 



The first line of evidence follows from the nature of the reactive dead para- 

 mecia. These form only the mating reaction union with living mates. They 

 fail to associate with the living animals in more intimate fashion or form the 

 secondary or tertiary types of union described in figure i. In the absence of 

 these other types of union it may be assumed that any special substances 

 responsible for such unions fail to interact. Clearly, then, interaction of the 

 mating type substances must initiate activation, or some completely unknown 

 and unsuspected substances must interact to initiate the series of activation 

 changes. The first of these possibilities seems the more likely in view of the 

 fact that neither the mating reactivity nor activating action of the dead para- 

 mecia is destroyed by such harsh and diverse agents as formalin and picric 

 acid. 



A second line of evidence supporting the view that mating type substance 

 interaction initiates activation in Paramecium resulted from an analysis of a 

 mutant stock of Paramecium aurelia (66, 67). These mutant, 'can't mate,' 

 (CM) animals give the initial agglutinative mating reaction with both CM 

 and normal animals of complementary mating type. However, they never 

 undergo the subsequent changes of conjugation. They cannot be activated 

 by sexual association with complementary CM or normal animals. Apparently 

 some block, the CM block, prevents activation from proceeding beyond the 

 intial stages in CM animals (fig. 3). Nevertheless, these mutant, blocked ani- 

 mals, whether alive or dead, can specifically activate normal, non-mutant 

 animals of complementary type. Since the CM animals can activate normal 

 animals, they clearly possess the activation initiating mechanism at least so far 

 as their ability to activate normal mates is concerned. 



Finally, since the CM animals possess the activation initiating mechanism 

 and since they can unite with mates only in the mating reaction, initiation 

 of activation must be related to the mating reaction. It follows from this that 

 interaction of mating type substances initiates activation in conjugating para- 

 mecia. 



The mutant CM animals are of further interest. Like non-mutant P. aurelia, 



