286 SEX IN MICROORGANISMS 



one might eventually achieve a biochemical description of the activa- 

 tion-initiating mechanism. This biochemical approach began with 

 Warburg's (1908) demonstration of the increase in respiration that 

 follows activation of the sea urchin egg. Although a great deal has 

 been learned about the biochemistry of the unfertilized as compared 

 with the activated egg, and although the approach has even been ex- 

 tended to fertilization in ciHates (Boell and Woodruff, 1941), it is 

 still largely in the descriptive stage. Even in the most thoroughly 

 studied form, the sea urchin, general agreement on metabolic path- 

 ways has yet to be reached (Rothschild, 1951a; Cleland and Roths- 

 child, 1952). Evidently, then, it will be some time before this ap- 

 proach to the problem may be expected to yield the desired answers 

 to the fertilization problem. 



The second approach to the problem of the activation-initiating 

 mechanism is through the medium of artificial parthenogenesis. Arti- 

 ficial parthenogenesis was first achieved with the silkworm egg 

 (Tichomiroff) in 1886. However, it was not until the turn of the 

 century, with Loeb's (1899) experiments on the sea urchin tgg, that 

 this field of investigation showed real promise. During the early part 

 of the century it was anticipated that parthenogenesis would provide 

 the key to the mechanism of action of the sperm in fertilization. How- 

 ever, a very large number and variety of effective physical and chem- 

 ical agents were soon discovered, and as yet no common factor has 

 been found among them. Parthenogenesis has made the important 

 contribution of demonstrating that the egg contains within itself all 

 essentials for development and has given substance to the stimulus or 

 trigger concept of sperm action, but as yet no comprehensive scheme 

 to explain parthenogenesis or to relate it to sperm activation has been 

 forthcoming. 



A third approach to the activation problem involves the investi- 

 gation of cell extractives and substances liberated spontaneously by 

 gametes or protozoa. This field is an attractive one because the effects 

 obtained are frequently striking and, like the biochemical studies, the 

 results are interesting in their own right whether or not they contri- 

 bute to an understanding of fertilization. The usual rationale here is 

 to attempt to ascribe a role in fertilization to such substances once 

 they arc obtained. The most thoroughgoing application of this 

 approach is to be found in the studies on the sperm and egg isoagglu- 

 tinins, fertilizin and antifertilizin. But, even after forty years of pain- 



