STUDIES OF FERTILIZATION 387 
b. An abortive karyokinetic figure may form with the egg 
nucleus alone, (Ziegler and Boveri on sea-urchins). 
c. In some eases at least two cleavages may result, (Boveri 
on sea-urchins). Boveri’s so-called ‘partial fertilization, is really 
full fertilization, so it does not come in this series. 
The difference in reaction of the egg-cytoplasm to its own 
nucleus and to the introduced part of the spermatozoon can be 
explained on only one of two grounds; either in the general sense 
of Boveri’s theory on purely morphological grounds, or on the 
ground of a chemical difference, presumably of sexual origin, 
between the egg on the one hand and the sperm on the other. 
The latter form of interpretation seems to me preferable because 
it is a physiological interpretation which takes cognizance of the 
sexual factor in fertilization. 
Boveri's theory of fertilization rests on the identification of the 
centrosomes of the sperm aster with a definite formed element 
(centrosome) introduced into the egg by the spermatozoon; 
but it has never been demonstrated in any case in all the liter- 
ature on the subject of fertilization that the centrosomes of the 
cleavage spindle, or indeed of the sperm aster, are derived from 
any definite formed element of the spermatozoon. Boveri 
himself admits this in his sixth cell study (07, page 266); and so 
long as definite proof of the continuity of the so-called sperm- 
centrosomes from the spermatid up to the formation of the first 
cleavage spindle is lacking, all of Boveri’s observations are open 
to another interpretation than the one he has given; to the inter- 
pretation, namely, that the sperm asters represent a reaction of 
the egg cytoplasm to a male element, or at least a foreign element, 
represented for the most part by the sperm nucleus. 
The biological analysis of fertilization seems to me to rest now 
upon the problem of the origin of the sperm aster in the egg. 
More crucial evidence is needed on this point, and I do not believe 
that any refinement of cytological technique will give the result. 
Experimental evidence is needed; either, as Boveri (’88) sug- 
gested, the introduction of a non-nucleated spermatozoon in the 
egg to prove whether or not the sperm asters would arise without 
the nucleus and fertilize the egg, or the introduction of only the 
