THF. PHYSIOLOGY OF FERTILIZATION IN CILIA IIS 305 



tain "killer" fluids may bear upon this question (Chen, 1945; Jacob- 

 son, 1948; Preer, 1948). The killer fluids in question induce activation 

 with and without pairing in certain "sensitive" stocks (see further 

 under parthenogenesis). The description (Chen, 1945) of the result- 

 ing pairs suggests that pairing involves a holdfast union. Therefore 

 it is possible that, in these exceptional pairs, holdfast substances are 

 produced in response to the activating action of the killer fluids and 

 that the animals unite directly through holdfast substance interaction. 

 The fact that pairs form only after fluid and animals have been mixed 

 for some time favors this view. If this explanation should prove cor- 

 rect, the mating reaction could not be a prerequisite for holdfast 

 unions, and failure to find holdfast unions in autogamy could best be 

 explained by assuming that holdfact substances are not formed in this 

 process. Since CM animals do not form holdfast unions, holdfast 

 substance formation must arise beyond b (Fig. 2). Thus the argument 

 presented here suggests that holdfast substance formation arises be- 

 tween b and c. 



Loss of Mating Reactivity. This reactivity occurs in conjuga- 

 tion, its natural and experimental variants, and in natural autogamy. 

 No statement is available regarding loss of mating reactivity in re- 

 sponse to "killer fluids" and no case of activation without loss of 

 mating reactivity has been reported. However, mating reactivity can 

 be regained in a remarkably short time after conjugation, as Diller 

 ( 1 942 ) and others have shown. Loss of mating reactivity does occur 

 in certain abnormal material (P. biirsaria) which fails to undergo 

 the normal nuclear cycle and in which conjugants separate prema- 

 turely (Chen, 1946d). Furthermore, Tartar and Chen (1941) found 

 that Uving enucleate fragments (P. biirsaria) lost mating reactivity 

 after clumping for a short time with animals of opposite type. From 

 these observations it appears that loss of mating reactivity can follow 

 directly from the mating reaction and is not dependent upon other 

 activation phenomena. It may be sequential to but not dependent 

 upon holdfast substance formation, since it occurs in natural au- 

 togamy. Thus it might arise between c and d in Fig. 2, Paroral cone 

 formation, macronuclear breakdown, and meiosis could be sequential 

 to loss of mating activity. 



Paroral Cone Formation. Observations on paroral cone forma- 

 tion are limited. Hertwdg (1889) clearly described these structures in 

 conjugants. They are formed also in pseudo selfing pairs (Metz, 



