1887] i The Significance of Sex. 41 
widely separated cells. When the germinal dot enters upon its 
activities as anucleus it passes through the stages of differen- 
tiating a reticulum and nucleoli of different kinds in itself, as we 
shall see under haryokinesis. 
(c) THE SPERMATOZOON. 
When the last division of the spermatocytes has taken place, 
the nucleus is practically ready for conjugation; hence, that its 
chromatin may meet the chromatin of the ovum, the secondary or 
achromatic structures of the nucleus transform themselves to- 
gether with the cytoplasm (which seems to play a more passive 
part), into the suitable mechanism for effecting the transfer. In 
most cases the resulting form is filamentous, and has a spiral 
structure in some part. (See Figs. 66-93.) In such highly com- 
plex spermatozoa we may distinguish the following parts: 
An outer membrane, which is perhaps the relic of the cell-mem- 
. brane. A head-cap, posterior to which lies the chromatin. An 
axial filament, which may be taken as a sort of skeleton. (See Fig. 
78, e.) Finally, there isa medullary sheath, best, sometimes only, 
developed in the “eck” or middle piece of the spermatozoon. 
This sheath is often composed of two or three bands that have 
been spirally twisted in opposite directions around the axial fila- 
ment. Often one of the three is free and hung by a delicate 
- mesentery, and thus may propel the spermatozoon like a screw. 
In the development of these parts, we first see the nucleus change 
its shape and become homogeneous, then the axial filament is seen 
stretching away from the nucleus and pushing the cytoplasm 
before it posteriorly as the nucleus does at the anterior end. The 
achromatic part of the nucleus is usually present as a paranucleus 
(see karyokinesis, Fig. 123, also Fig. 81), which in some cases is 
directly converted into the medullary sheath. Paranuclei (or 
granules) of a different sort are often present, and may have some- 
thing to do in building the axial filament. All growth takes 
place in the neck just behind the head; and from this point the 
tail end is gradually pushed out as a completed structure. These 
accessory parts having accomplished their work of transferring 
the chromatin, which is to form the male pronucleus, are lost 
or dissolved in various ways. The chromatin is the essential sub- 
stance, as we shall learn under “ Fertilization? 
The d t t in the special cases may be 
E E 
