302 SEX IN MICROORGANISMS 



selfing" pair formation), meiosis, and macronuclear breakdown in 

 normal animals. These relations were discovered in mixtures of living 

 and dead animals and are so summarized in Table IV, but they apply 

 equally well to living-living mixtures of animals of opposite type. 



Since the CM animals cannot form holdfast or paroral unions, it 

 is reasonable to conclude that the CM animals lack any special hold- 

 fast or paroral cone substances. Nevertheless, the CM animals can 

 activate normal animals, and therefore they possess the activation- 

 initiating mechanism. Consequently, interaction of holdfast or paroral 

 substances is not essential for activation. The only other known sub- 

 stances which could interact to initiate the various changes in con- 

 jugation, then, are the mating-type substances. Therefore, activation 

 in Parmnecium must result from interaction of some as yet unknown 

 substances, or from interaction of the mating-type substances. The 

 latter alternative is accepted as the simpler hypothesis. 



Interrelation of the Activation Phenomena 



The interrelationship of the events which follow the activating 

 reaction have not been investigated as thoroughly as their importance 

 warrants. However, some information is available and more may be 

 expected in the future. 



Since the subsequent events in conjugation follow in an orderly 

 sequence from the initial reaction, it is reasonable to suppose that 



HOLDFAST SUBSTANCE FORMATIONl?) 



INTERACTION CM NATURAL AUTOGAMY / 



OF SURFACE • I X INITIATED HERE / ^ 



• / / » LOSS OF MATING ACTIVITY 



(MATtNG TYPE?) 

 SUBSTANCES 



> > > > — * -> -» PARORAL CONE FORMATION 



\^- 



\ ^ MEIOSIS 



\ 



MACRONUCLEAR BREAKDOWN 



a b C d e 



Fig. 2. Scheme for activation in Farainecium. 



(a) Initiating reaction (mating-type-substance interaction?) in sexually induced acti- 

 vation, (b) CM block, here assumed to lie "internal" to the initiating reaction, (a), 

 (c) Position where chain is activated in natural autogamy, (d) Breakup of main 

 activation chain into side reactions leading to (e) the various end effects of acti- 

 vation. 



