INTRODUCTION 3 
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. O. 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. 7 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 
