C. B. METZ 25 



ditions, by sex substances, hormones, etc., or a combination of 

 these factors. These interesting prehminaries to fertihzation are 

 beyond the scope of this review. Only those substances derived 

 from and directly affecting the gametes themselves will be con- 

 sidered here. 



The more spectacular and more readily investigated sex sub- 

 stances of this sort appear in the natural fluids containing the 

 gametes. Theii' presence in such fluids indicates that they are 

 available under natural conditions and their action upon gam- 

 etes, especially those of the opposite sex, constitutes strong evi- 

 dence that they function in fertilization. In the demonstrable 

 absence of sex substances in natural fluids, the interaction of 

 gametes at fertilization may again be attributed to more or less 

 specific sex substances, but in this case the agents must be insolu- 

 ble in the fluid medium, firmly bound to the gametes or both. 

 The sex substances of Paramecium are exclusively of this type 

 (Metz, 1954a), but comparable agents have not been examined 

 extensively in other organisms. Upon proper analysis such insolu- 

 ble or bound sex substances may be found to occur widely, and 

 an analysis of their role in fertilization may contribute much to 

 our understanding of the physiology of fertilization. 



Aside from the agents that appear in the media surrounding 

 the gametes, agents of various sorts may be obtained by more or 

 less elaborate extraction of gametes. A number of such agents 

 have definite action upon gametes of the same or opposite sex, 

 but the question of their role in fertilization is not so readily set- 

 tled for the agents may not be available under natural physiolog- 

 ical conditions. Indeed they may be artificial products of the 

 extraction procedures. The basic proteins (protamines, histones) 

 obtained by acid extraction of sperm are notable recent examples 

 of this. 



Agents from Eggs 



The simplest preparation from eggs is the supernatant water 

 in which eggs have been suspended. Such egg water may be ex- 

 pected to be charged with soluble and readily diffusible agents 

 from the eggs. The most thoroughly studied of these agents is 



