STUDIES OF FERTILIZATION 383 
pared with the former, the chief difference observable by cytologi- 
cal methods is the entire absence of the achromatic part of the 
karyokinetic figure. The differences in later stages may be 
conceived as secondary effects of this defect or of the conditions 
determining such defect. When eggs are caused to mature by 
centrifuging the process begins as in normally fertilized eggs by 
the breaking down of the cortical layer and formation of the 
jelly; the germinal vesicle ruptures and the two maturation 
divisions follow. After the completion of the maturation 
the chromosomal vesicles of the egg nucleus usually fail to unite 
perfectly, and in a little while they separate and scatter in the 
cytoplasm without formation of chromosomes, so that each egg 
appears to possess a considerable number of small nuclei. In a 
few cases the first indications of chromosome formation may be 
observed in the vesicles shortly after maturation but not later. 
Subsequently the chromosomal vesicles appear to dissolve in the 
cytoplasm liberating small chromatic nucleoli. 
The partial stimulus of the spermatozoon is thus somewhat 
more effective than the mechanical shock of centrifuging, though 
both produced the same initial changes, apparently equally well. 
This may possibly be due to entrance of a small amount of matter 
from the spermatozoon; for at the critical period the perforator- 
ium of the sperm has penetrated the membrane and is imbedded 
in the entrance cone. 
D. General discussion 
The general conclusion that the function of the spermatozoon 
in the stimulus to development involves at least two factors 
has already been clearly stated by Boveri (’07) and Loeb (’09b): 
According to Loeb, one factor is the cytolysis of the ‘‘very thin 
cortical layer of egg’’; but while this stimulates development, 
the latter is often abnormal and therefore usually comes to a 
halt. A second process is necessary to ensure more normal and 
lasting development (Loeb ’09b). Apparently Loeb is not very 
clear concerning the nature of the second factor, but is inclined 
to regard it as inhibiting the cytolysis which he conceives to be 
