THE PHYSIOLOGY OF FERTILIZATION IN CILIATES 



303 



rhcv arc related to this initial event throug-Ji a predetermined chain of 

 reactions. One form of such a sclieme has been presented graphically 

 by Metz (1948), and his figure is reproduced here (Fig. 2). Support 

 for this type of scheme is derived from the study of abnormal or 

 mutant stocks. Of these the CM stock in P. aurelia has been examined 

 most thoroughly (Aletz, 1948; Metz and Foley, 1949). it. will be 

 recalled (Table I\ ) that the CM animals can activate normal animals 

 but that thev cannot themselves be activated by sexual means. Appar- 

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

 much beyond the initial stages in the CM animals. 



A< — >a 



—>-»-» 



I I 

 I I 



I< — >R- 



Rk—^V 



I I 



I b I 



Fig. 3. Two possible activation-initiating mechanisms. 



Each series of arrows represents the main activation chain in one conjugant. (a) 

 Simultaneous activation of conjugants by interaction of a single pair of surface 

 substances, (b) Simultaneous activation of conjugants by interaction of two pairs 

 of surface substances. 



The CM block is of particular interest because the CM animals 

 regularly undergo natural autogamy. Two important facts may be 

 deduced from this: (1) Activation in conjugation (and its experi- 

 mental variants) and natural autogamy are initiated through different 

 mechanisms. This will be discussed under parthenogenesis. (2) The 

 activating system "internal" to the CM block is intact. This suggests 

 that the block may be a relatively simple deficiency such as the lack 

 of an enzyme or essential substrate. 



At least two possibilities exist for the position of the block. These 

 depend upon the nature of the activation-initiating mechanism. If 

 activation is initiated simultaneously in both mates by interaction 

 of a single pair of substances (Fig. 3a), the CM block must lie internal 



