172 
spermatogenesis of other invertebrates) to form four 
spermatids.  Disc-shaped masses of spermatids are thus 
produced, the thickness of which is equal to about one- 
fourth the diameter of the face (fig. 65). In each mass 
there is a central cavity containing the remains of the 
blastophore, a small quantity of a shghtly fibrous 
coaguluin being present. Hach spermatid undergoes no 
further division, but is gradually transtormed into a 
spermatozoon. An early stage of this transformation is 
seen in fig. 64. The nucleus is an oval compact body at 
one end of which is a small conical mass of protoplasm, by 
which the spermatid is attached to the blastophore. This 
becomes the apical body or acrosome of the spermatozoon. 
At the opposite end of the nucleus there is a clear sub- 
‘ 
stance from which the “middle piece” of the sperma- 
tozoon is apparently largely derived, and following this 
is the rest of the protoplasm, which is being drawn out to 
form the tail of the sperm. When shed into the sea the 
ripe spermatozoa soon become free from the blastophore 
and move by means of the tail, which appears to be a 
somewhat stiff filament capable of comparatively limited 
movements (fig. 66). A ripe spermatozoon is about “058 
mum. long, it has a curiously shaped head -04 mm. long, 
and a long slender tail (‘054 mm. long). The nucleus forms 
the greater part of the head of the sperm. At one end is 
the apical body forming a cap shghtly sub-divided by a 
median groove. It is by means of this apical cap that the 
sperm is later attached to the egg with which it is about to 
unite. At the other end of the nucleus is the middle 
piece which is apparently notched behind to receive the 
basal part of the tail. tipe sperms may usually be 
obtained about the end of February or the beginning of March. 
The genital products, instead of floating freely im the 
ceelomic fluid, often accumulate in the space between the 
