January 14, 1916] 



SCIENCE 



49 



attained a more or less definitive solution 

 of the morphological relations of egg and 

 spermatozoon in the fertilization process. 



The cytologist working with chromosomes 

 and the geneticist with Mendelian factors 

 have traced maternal and paternal elements 

 through the life history in a manner very 

 satisfactory to certain classes of biologists, 

 however repugnant to others, so that we 

 are beginning to see how certain strands of 

 the web of life cross the gap of successive 

 generations. It remains for the biology of 

 the future to elucidate the chemical foun- 

 dations of chromosome behavior and to 

 identify the Mendelian factors in these 

 chemical foundations. 



The problems of the immediate reaction 

 of the reproductive elements and the phys- 

 iology of fertilization are not touched by 

 this morphological analysis, though they 

 had been present in the minds of investiga- 

 tors from the beginning. The experimen- 

 tal investigation of these problems dates 

 from Spallanzani, as we have seen, but they 

 did not become dominant until the morpho- 

 logical problems of fertilization were in an 

 advanced stage of solution. They consti- 

 tute, however, the more immediate prob- 

 lems of fertilization, considered in a re- 

 stricted sense. 



We have had two lines of attack since 

 the studies of 0. and R. Hertwig pub- 

 lished 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 at- 

 tempt to deal with the latter, which con- 

 stitute one of the most interesting and sug- 

 gestive 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 artificial parthenogenesis that no single 

 physical or chemical agency suffices to ini- 

 tiate development in all eggs, and that 

 when the various agencies effective in all 

 the successful 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 must be the effective one, 

 but no common factor has been found, or 

 can be found, in the agents themselves ; the 

 only common factors are in the reproduc- 

 tive cells. This leaves the method of par- 

 thenogenesis in the same position as the 

 method of analysis, that is in the position 

 of determining what are the changes in the 

 egg itself that initiate development, and 

 what is the nature of their dependency 

 upon the external agent or spermatozoon? 

 The answer to these questions can not pro- 

 ceed exclusively from parthenogenetic stud- 

 ies, though to the extent that the same ques- 

 tions are involved parthenogenesis and fer- 

 tilization studies must furnish the same 

 answer. But there are obviously funda- 

 mental problems of fertilization that can 

 not be touched by methods of artificial 

 parthenogenesis. 



The conditions to be fulfilled in fertiliza- 

 tion involve not only penetration of the 

 spermatozoon, or some part of it, into the 

 egg, but also reaction between the two, 

 which is evidenced by the behavior of both 

 partners; for it is possible for a spermato- 

 zoon to penetrate an egg and no reaction to 

 be evidenced in the behavior of either the 

 egg or sperm; as when immature eggs are 

 penetrated by mature spermatozoa. We 

 may therefore speak of a fertilization re- 

 action when the behavior of both partners 

 indicates that the process is proceeding nor- 

 mally. Fertilization has its quantitative 

 aspect, and the reaction may be complete 

 or exhibit varying degrees of incomplete- 



