300 SEX IN MICROORGANISMS 



various manifestations of activation must be initiated by one of the 

 following mechanisms: (a) direct interaction of fixed surface sub- 

 stances, (b) transfer or diffusion of substance (s), (c) a combination 

 of surface interactions and transfer of substances. 



As Sonneborn (1949) points out, none of these possibilities has 

 been excluded by direct experiment. Nevertheless, the constitution of 

 the dead animals which are capable of activating living animals and 

 the nature of the reaction between them render the first of these 

 possibilities highly probable. The reaction between living and dead 

 animals is a superficial one to the extent that mates can be separated 

 mechanically at any time during their union. As seen in Table V 

 certain of the effective killing agents are strong fixatives. After treat- 

 ment with these agents the dead animals must be washed repeatedly 

 to remove the killing agent. In this process of killing and fixation, any 

 substance that is appreciably soluble in water would be removed from 

 the dead animals. Thus, any essential, diffusible agent would nec- 

 essarily be a rather special, relatively water-insoluble substance. 

 Unfortunately, dead animals that have been thoroughly extracted 

 with lipid solvents have never been tested for their ability to activate 

 living animals. However, it is not unlikely that the activating proper- 

 ties will withstand such treatment, since lyophilized animals (P. aiire- 

 lia) give good mating reactions after extraction with absolute acetone, 

 ether, chloroform, or benzene (Metz and Fusco, 1949). In view of 

 these considerations it appears highly probable that the activation-ini- 

 tiating substances are surface substances and that the essential activat- 

 ing reaction (s) is a reaction between these surface substances. The 

 question then arises as to what surface substances interact to initiate 

 activation. Experimentally, the problem is most readily approached 

 from a consideration of the three types of union that occur in con- 

 jugation. These are: (1) the mating reaction union, (2) the holdfast 

 union (Fig. la), and (3) the paroral union (Fig. lb). 



Of these several types of union the last is clearly unnecessary 

 for activation, as shown by the following three observations: (1) 

 Paroral union occurs after the holdfast union is formed, mating reac- 

 tivity is lost, and meiosis has begun. (2) Dead animals which are 

 capable of activating living animals do not possess paroral cones. (3) 

 The third animal in conjugating "threes" (Chen, 1940a, 1946b; Metz, 

 1947) is usually joined to the primary pair only by its holdfast region. 



