STUDIES OF FERTILIZATION 433 
able interpretation of this picture, although it was taken from a 
preparation killed fifteen minutes after centrifuging; further pene- 
tration following centrifuging was probably prevented in this case 
by the large oil drops which were driven around the enteringsperm. 
3. Penetration of injured spermatozoa 
Injured or partial spermatozoa may enter the egg, demon- 
strating that penetration of the spermatozo6n, after attachment 
is once secured, is an active function of the egg and not at all of 
movements of the sperm itself, if any further evidence is needed 
on this point. But if the entire spermatozoén be removed, the 
cone remains superficial and does not penetrate; at least I have 
repeatedly found it in a superficial position fifteen to thirty or 
more minutes after the normal time of penetration, and I have 
never found it actually penetrated unless accompanied by all 
or a part of the sperm nucleus. 
The evidence for the entrance of injured or partial sperm heads 
is furnished by cytological study of eggs fixed at definite periods 
after centrifuging. Particularly clear evidence is furnished by one 
series of nine stages fixed fifteen minutes after centrifuging. In 
the fifth stage of this series centrifuged fifty minutes after insemi- 
nation and fixed fifteen minutes later, early stages of penetration 
are very abundant, and among them are some that show unequi- 
vocal evidence of being partial sperm nuclei, for there are numer- 
ous cases in which an injured part of the sperm head is left out- 
side on the membrane and thus guarantees the partial nature of 
the sperm nucleus within. Some such eases are illustrated in 
figs. 27 to 32. 
Fig. 27 shows a case killed fifteen minutes after centrifuging 
in which the connection between the internal and external parts 
of the spermatozo6n is entirely broken. The partial sperm nu- 
cleus within the egg and the cone have already begun to rotate. 
In fig. 28 we have a similar case, except that there still remains 
a delicate connection between internal and external parts of the 
spermatozoon, and this condition would lead, by rupture of this 
connection, to the condition shown in fig. 27. It would seem, 
