STUDIES OF FERTILIZATION 379 
explain why eggs in which the sperm nucleus was absent segmented 
but on the contrary, why certain eggs that possessed the sperm 
nucleus failed to segment, which is a very different thing. There 
is no evidence that any egg in which the sperm nucleus was absent 
succeeded in dividing. 
The general conclusion that removal of the spermatozoon at the 
times noted in the experiments involves incomplete fertilization, 
is sufficiently demonstrated by the results. 
Let us call the stage at which the spermatozoon is eliminated 
in the greatest proportion of eggs, the critical period. The 
exact number of minutes from the time of mixing eggs and sperm 
to this stage varies of course through the season, owing to the 
variations of temperature. Moreover, it is not exactly deter- 
mined in all experiments, for in some the stages of centrifuging 
fall on either side of it. This being understood, we may note that 
in eight experiments the critical period occurred at from 25 min- 
utes to 40 minutes after fertilization. This is quite a wide 
variation, but when the time is represented asa fraction of the 
entire period between fertilization and the first cleavage, it 
is found that in all cases the period up to the critical period is 
between 27 and 33 per cent of the total time up to the first cleav- 
age. It is obviously a corresponding stage in all cases, for the 
observed differences fall within the chances of error, viz: that the 
critical period is hit exactly in only very few experiments, and 
that the time of beginning of the first cleavage must be stated 
rather arbitrarily on account of the variation in rate of individ- 
ual eggs. . 
The critical period occurs shortly before the penetration 
of the spermatozoon into the egg. We noted in the first part 
of this paper that the penetration of the spermatozoon is extreme- 
ly gradual; my observations on this point, both from the study of 
the living egg and also of sections, show that it requires forty to 
fifty minutes for the head of the spermatozoon to disappear 
through the membrane. 
As the most critical period comes in the great majority of 
experiments from thirty-five to forty minutes after insemina- 
tion, it is obvious that the spermatozoon is in some way prevented 
JOURNAL OF MORPHOLOGY, VOL. 22, No. 2 
