THE PHYSIOLOGY OF FFRTIMZATION IN CILIATES 287 



stakiniT research, no invcsrigaror has presented convincing evidence 

 that these substances are essential for fertih/ation. 



The fourth avenue of attack follows logically from the preceding 

 one and, in fact, represents a more systematic development of it. This 

 approach involves the partial or preferably complete isolation of the 

 activation-initiating mechanism and a study and characterization of 

 its parts. So far this method has not been employed with success on 

 metazoa, in spite of many attempts to activate eggs with sperm 

 extracts or dead sperm. However, this method has met with rather 

 striking success in the ciliate ParaiJiechnu. Since this approach and 

 the associated preceding one are concerned with various sex sub- 

 stances, a further discussion of these agents will be presented before a 

 comprehensive account of fertilization in Paraniec'mm and other 

 ciliates is given. 



SEX SUBSTANCES AND MATING SUBSTANCES 



Beginning with Frank Lillie's (1913, 1914, 1919) now classic 

 studies on fertilizin in the sea urchin and the annelid worm, Nereis, 

 the most fruitful approach to many of the problems of fertilization 

 has been through an analysis of specific interacting substances of sex 

 cells. Indeed, these have produced the only comprehensive, though 

 now outmoded, theory of fertilization, namely, Lillie's fertilizin 

 theory. Such sex substances have been demonstrated or their presence 

 inferred in many organisms, both plant and animal, unicellular and 

 multicellular. In ciliates these substances are produced only when the 

 organisms are in the sexual or mating condition and are capable of 

 fertilization. These agents, the mating substances, have specific action 

 upon ciliates of complementary sex or mating type and presumably 

 perform some function in fertilization. Correspondingly in metazoa, 

 agents may be obtained from eggs and sperm which specifically affect 

 gametes of the species. Again these agents are produced (presumably) 

 by the gametes themselves and are believed to function in fertilization. 



In both groups of organisms the more spectacular and more 

 readily studied sex agents are those which appear in the fluid contain- 

 ing the gametes or sexually reactive protozoa. Such water-soluble 

 substances are actively produced and secreted or dissolve passively 

 from the cell or its surface into the fluid. Among ciliates the only 

 well-established example of such diffusible mating substances is in 



