INTRODUCTION 



experiments through which it is possible to cause, by chemical 

 means, the unfertilized eggs of various animals to develop into 

 larvae. 



Such a study could not be undertaken without applying 

 the methods of experimental and, more especially, quantitative 

 research. As long as the spermatozoon was the only means of 

 calling forth the development of the egg it was impossible to 

 undertake a physicochemical analysis of this process. The 

 work on artificial parthenogenesis, i.e., the substitution of well- 

 known physicochemical forces for the spermatozoon, made 

 such an analysis possible. 



The older so-called theories of fertilization were merely 

 metaphors. The egg was compared to a clock, and it was said 

 that the spermatozoon set this clock in motion. Others said 

 that the spermatozoon communicated to the egg a pecu- 

 liar mode of vibration, and still others maintained that the 

 spermatozoon imparted a "stimulus" to the egg. "Stimulus" 

 is a technical term, but scientific problems are not solved by 

 mere acts of nomenclature or rhetoric. 



With the rise of cytology more definite ideas in regard to the 

 mechanism of fertilization were expressed. 0. Hertwig defined 

 fertilization as the fusion of the sperm nucleus with the egg 

 nucleus. While this fusion has a bearing upon the transmission 

 of the paternal characters to the offspring, it does not give us 

 any insight into the nature of the forces by which the egg 

 is caused to develop. Hertwig's definition is also incorrect, 

 as was clearly demonstrated by Boveri who found that the 

 fusion of the egg and sperm nucleus had nothing at all to do 

 with the causation of the development of the egg. For he was 

 able to show that an egg deprived of its nucleus can actually 

 develop into an embryo if a spermatozoon enters it. In this 

 case no union of two nuclei takes place. 



Boveri replaced Hertwig's definition by a hypothesis that 

 was more in accordance with the facts. According to him, the 



