STUDIES OF FERTILIZATION 389 
therefore, we are to hold to the theory of Boveri in its literal 
sense, we must believe that there are different kinds of inhibition. 
However, it is, I believe, simpler and more logical to hold that 
the inhibition differs only in intensity at these various stages; 
and this point of view seems to be supported by the fact that the 
same stimulus which at a lower intensity will cause only matur- 
ation to take place in Chaetopterus, at a higher intensity will 
cause differentiation also to proceed, though in this case without 
cleavage. Boveri (’07), however, holds that there are different 
kinds of inhibition, that the postulated degeneracy of the egg- 
centrosomes after maturation is in a sense the more primitive, 
and that other kinds have been secondarily acquired, a point of 
view that gives a more or less definitely teleological aspect to 
the question. 
From a physiological point of view we might inquire, what are 
the conditions that cause the postulated sudden degeneration 
of the egg-centrosomes? Such a condition if found, would be 
nearer the fundamental cause of inhibition of the egg and it might 
turn out to be the same cause that conditions in so many cases 
an earlier arrest of activities in the egg. 
The experiments on artificial parthenogenesis are sometimes 
regarded as involving the entire problem of fertilization. But 
if it be true, as many believe, that biological fertilization, (if I 
may be pardoned such an expression) is fundamentally a sexual re- 
action, then the physico-chemical analysis of fertilization must 
compass the entire problem of sex, which is much wider than the 
problem of parthenogenesis. The physico-chemical analysis 
of fertilization has dealt, up to the present exclusively, with the 
latter problem, and for this reason the earlier title of such studies 
‘artificial parthenogenesis’, seems to me much more fitting than 
‘chemical fertilization’ which is sometimes loosely used. From 
the zoological point of view, at least, parthenogenesis and fertil- 
ization are not interchangeable functions. There is a factor 
present in fertilization which is absent in parthenogenesis, and 
the latter is never the exclusive mode of reproduction among 
animals. The biological analysis of fertilization therefore 
involves problems that do not occur in the physico-chemical 
analysis of parthenogenesis. 
