56 ARTIFICIAL PARTHENOGENESIS AND FERTILIZATION 
a single criterion which allows us to decide whether an agent 
has an effect upon the egg similar to that of the entry of the 
spermatozoon; this criterion is the development of the egg 
into alarva. The mere initiation of cell division is not sufficient 
since, for example, cell divisions occur in the cases of the growth 
of tumors and galls, which do not lead to the formation of a 
larva. This distinction between a cell division which forms the 
basis of normal development and growth and one which leads 
to the formation of pathological products is also of practical 
importance. 
It is not our purpose in this book to report all the investi- 
gations upon artificial parthenogenesis. We shall rather con- 
fine ourselves to those experiments which help us to obtain an 
insight into the physicochemical character of the process of 
development. We will start with the experiments upon sea- 
urchin eggs, which appear to be best suited to such investiga- 
tions. The forms of sea-urchins with whose eggs I have worked 
are Arbacia of the Atlantic Ocean (at Woods Hole) and Strongy- 
locentrotus purpuratus and franciscanus of the Pacific Ocean (at 
Pacifie Grove). 
The eggs of both these forms develop only when incited 
thereto either by sperm or by the chemical methods hereafter 
to be described. 
