1 HE PHYSIOLOGY OF FI.R TILIZA TION IN CILIATES 285 



PROBLEMS AND METHODS 



The prinv.irv problem in the physiology of fertilization is the 

 problem of the activation initiating meclianism. Associated with this 

 primary event are certain other phenomena which constitute special 

 problems in themselves. Thus: (1 ) the metazoan gamete or the ciliate 

 must be sexually ripe or reactive before fertilization can occur; (2) 

 some degree of attachment or union of the reacting cells would ap- 

 pear to be essential for their interaction; (3) fertilization is univer- 

 sally characterized by a high order of specificity; (4) an astonishing 

 series of biochemical and morphological changes follows immediately 

 upon activation in most forms; and finally (5) passage of nuclear 

 material from one cell to another presupposes some degree of func- 

 tional fusion of cells. In passing it may be noted that some of these 

 problems also apply to several other major biological phenomena, 

 notably embryonic induction, host-virus relationships, and specific 

 tissue affinities. Although major interest has centered about the prob- 

 lem of the activation-initiating mechanism, a comprehensive account 

 of fertilization must provide not only for the initiating reaction itself 

 but for these associated phenomena as well. The second, third, and 

 fourth of these phenomena, as well as the activation initiating mecha- 

 nism, \v411 be considered in this review. 



The various morphological, physiological, and biochemical 

 changes that occur at fertilization all follow in a very precise and 

 orderly sequence under favorable circumstances. This has led to the 

 view that most, if not all, of these many changes are interrelated, that 

 they all proceed from a few or even a single event — a chemical reac- 

 tion between the interacting cells. Considered in this fashion, it would 

 not seem excessively difficult to identify the activation-initiating 

 reaction or reactions. However, this and the associated problems have 

 been studied for over fifty years and many competent investigators 

 have directed their attention toward them, but no comprehensive 

 theory based on substantial experimental data is yet available to de- 

 scribe them. Four general methods have been employed by various 

 investigators in their attempts to reveal the mechanism of fertilization. 

 These are outlined below. 



The first of these may be called the biochemical approach. By 

 studying the end effects of activation, particularly the physical and 

 metabolic effects, and by tracing these back to their initial causes. 



