HISTORY OF THE FERTILIZATION PROBLEM 21 



chromosome behavior and to identify the Mendelian 

 factors in these chemical foundations. 



The problems of the immediate reaction of the repro- 

 ductive elements and the physiology of fertilization are 

 not touched by this morphological analysis, though they 

 had been present in the minds of investigators from the 

 beginning. The experimental investigation of these 

 problems dates from Spallanzani, as we have seen, but 

 they did not become dominant until the morphological 

 problems of fertilization were in an advanced stage of 

 solution. They constitute, however, the more imme- 

 diate problems of fertilization, considered in a restricted 

 sense. 



We have had two lines of attack since the studies 

 of Oskar and Richard Hertwig published in 1887 really 

 initiated the modern period in the physiology of fertiliza- 

 tion. The one is a direct experimental analysis of the 

 fertilization process itself; the other is the attempt to 

 imitate the action of the spermatozoon by chemical and 

 physical agencies in short, the studies on artificial 

 parthenogenesis. I shall not attempt in the present 

 chapter to deal with the latter, which constitute one of 

 the most interesting and suggestive chapters in modern 

 biology, beyond attempting to define their relation to 

 the problems of fertilization proper. 



It was soon found in the course of studies on arti- 

 ficial parthenogenesis that no single physical or chemical 

 agency suffices to initiate development in all eggs, and 

 that when the various agencies effective in all the success- 

 ful experiments are assembled they constitute a fairly 

 complete list of agencies to which protoplasm in general 

 is irritable. The idea then arose that the common factor 



