position posterior to the nucleus. One is led to believe from study 
of this transformation that this change of position is due to mecha- 
nical causes; that it is drawn into its final position by the changes 
of the cytoplasm of the spermatid into the tail of the spermatozoon, 
and the consequent pressure from the cell membrane of the spermatid 
which becomes tightly drawn over the head of the spermatozoon. 
Cell membrane. A delicate cell membrane encloses the nucleus, 
with the centrosome and the Nebenkern. This membrane is best seen 
in cases where a slight separation has occurred between the nucleus 
and the Nebenkern of the mature spermatozoon, when this membrane 
is stretched but still holds them together. In case of some glycerine 
preparations of spermatozoa killed in osmic vapor, stained with Dn- 
LAFIELD’s Haematoxylin, subsequent examination showed that the 
spermatozoa had swollen and burst, a portion oi the nuclear sub- 
stance had escaped, leaving the membrane distinctly visible: but from 
other observations, particularly in cases where the centrosome had ~ 
become pushed out of the depression in the nucleus, I am led to . 
believe that this is more than the nuclear membrane; that a cell — 
membrane, probably the original cell membrane of the spermatid per- — 
sists in the mature spermatozoon. | 
Tail. As Pıcrer has shown, the tail is formed from the cyto- _ 
plasm of the spermatid. His account I can confirm for all the Echino- — 
derm groups. PICTET concluded that the tail is attached to the nucleus ; 
but the fact that upon treatment with those reagents which cause 
the nucleus to swell and burst, the tail is almost invariably left at- 
tached to the Nebenkern seems to point otherwise. It seems to me — 
more probable that the tail is united with the cell membrane which 
invests the spermatozoon. 
The points referred to above show with more distinctness that 
the spermatozoon is a typical cell, and that it has all the parts or 
organs of such: a nucleus, with nuclein and caryoplasma, and a mi- 
totic nuclear spindle (the Nebenkern); a centrosome; cytoplasm; a 
cell membrane. The discovery of the spermcentrosome shows a closer 
parity between the ovum and the spermatozoon. The fact that a 
centrosome is present in the spermatozoon increases the probability 
that a centrosome is present in every cell capable of mitotic division, 
and that it is to be regarded as a permanent organ of the cell. It 
seems too that something besides the nucleus proper penetrates the 
ovum in the fertilization process, though the centrosome may have 
