STUDIES OF FERTILIZATION 42] 
a narrow chromatin band as it enters through the minute aper- 
ture in the vitelline membrane. Figs. 9 a and 9 b (fifty-four 
minutes after insemination) show penetration completed and it 
will be noted that the middle piece remains external. 
Examining now the details of this process, every stage of which 
is found in the preparations, we may note: (1) The sperm nuc'eus 
enters through an aperture in the membrane much smaller than 
itself, and is hence drawn out into a strand. (2) The inner end 
of the entering sperm nucleus begins to swell, by absorption 
of fluid soon after its entrance has begun (figs. 6, 7, 8). (3) 
Striae appear in the protoplasm between the fertilization cone 
and the surface of the egg. (4) When penetration has actually 
begun (fig. 6) the distance, corresponding to the length of the 
perforatorium, which originally separated the chromatin of the 
spermatozo6n from the attachment granules in the fertilization 
cone has become very much reduced. The sperm nucleus is now 
almost or actually in contact with the fertilization cone (figs. 6, 
Sir 
If we inquire into the mechanism of this process, it is quite 
obvious that the initiative, so to speak, is on the side of the ovum. 
It is inconceivable that the spermatozoén should be the primum 
movens and push the fertilization cone before it into the proto- 
plasm without being coiled up or bent within the protoplasm. 
But, in the many instances observed, it is invariably as straight 
as shown in the figures. The fertilization cone indeed appears 
to actively penetrate, or to be engulfed by the egg protoplasm, 
and to draw the sperm nucleus after it through the narrow aper- 
ture in the vitelline membrane. 
The final stage of penetration is shown in figs. 9a and9b. The 
entire sperm nucleus is now within the egg cytoplasm, some dis- 
tance from the periphery, and it will be seen that the middle 
piece has remained without on the egg membrane. This is per- 
fectly characteristic, if not invariable, and the same observa- 
tion was made repeatedly on the living egg as already stated. 
The picture is perfectly clear as shown under a very high magni- 
fication in fig. 9 6, and not only is the middle piece plainly external, 
but there is no sign whatever of any sperm-component at the base. 
