380 FRANK R. LILLIE 
from entering the egg. The explanation is comparatively simple; 
the spermatozoon is imbedded in the jelly by which the egg is 
surrounded. When the jelly is first formed it is very viscid, and 
adheres to the eggs during centrifuging so that they mat together 
in the centrifuge. However, this stage passes and the result of 
centrifuging is then to separate the jelly from the eggs. In many 
eases the jelly carries off the attached spermatozoon with it. 
After penetration this can of course no longer happen. The curve 
of variation of the per cent of eggs centrifuged before penetration 
that fail to segment is due to the following factors: (a) At about 
twenty-five minutes the sperm head is sunk in a deep depression 
of the membrane and hence is less likely to be torn away by the 
jelly; (b) the change in consistency of the jelly presumably 
extends from without inwards; hence at first the innermost layer 
in which the spermatozoon is imbedded tends for a time (also 
presumably) to remain with the egg; (c) the time of penetration 
varies somewhat in any lot of eggs. These facts together would 
explain why the percentage of eggs that fail to segment after cen- 
trifuging rises to a maximum and sinks again to a minimum in 
the successive stages of centrifuging. 
The fact that centrifuging inhibits cleavage in a small per cent 
of the eggs from fifty to fifty-five minutes after insemination, 
leads me to suspect that in some cases the sperm may be destroyed 
after its penetration into the egg. In experiment 2, 1910, for 
instance, cleavage was inhibited in 25 per cent of the eggs centri- 
fuged fifty-three minutes after fertilization; in the control eggs 
killed at the time of centrifuging, penetration of the sperm is 
completed in the great majority of eggs, though it is found exter- 
nal still in a very few; it is difficult to estimate the per cent of the 
latter, but the impression is that it is less than 25 per cent. How- 
ever, it is impossible to confirm this, and I mention the matter 
here to call attention to the error in my first paper read before the 
Central Branch of the American Zoological Society (Abstract in 
Science, vol. 18, p. 36, May, 1910),'in which I stated that the de- 
struction of the sperm nucleus followed penetration. A renewed 
study of the penetration has proved that this is not the case, 
usually at least. 
