98 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
the spermatid into the spermatozoon, the centrosome is found close 
to the nucleus. The latter becomes spherical and expands into a 
vesicle, with the chromatin arranged chiefly around the periphery 
(pi. 16, fig. 125). The centrosome becomes unusually prominent at 
this time. The astral rays have disappeared; and the centrosome is 
now a comparatively large body occupying the center of a vacuole, 
which almost equals the nucleus in size (pi. 17, fig. 134). Its position 
with reference to the nucleus is extremely variable, except that it is 
always close to the nucleus. In the majority of cases, it occupies the 
anterior pole of the nucleus (pi. 17, figs. 132, 134, 136). But it may 
also be found to the right or to the left of it (pi. 17, figs. 133, 135, 
137), or even at the posterior pole (pi. 17, figs. 140, 141). Isolated 
cases like this are apt to be misleading as one might be easily led to 
conclude that the axial filament grows out from the centrosome at 
this point, and that later the centrosome is converted into the middle 
piece of the spermatozoon. 
I have already described the origin of the axial filament. Assuming 
a continuitv of the evtoreticulum with the nuclear reticulum for rea- 
«/ 
sons already given, there is no reason why the axial filament should 
not be connected with the centrosome even though the latter is located 
at the anterior pole. 
The variable position of the centrosome I have already explained 
as being due not to migration of the centrosome around the nucleus, 
but to lateral growth of the nucleus as it is reconstructed after the last 
maturation division. The subsequent history of the spermatozoon 
shows that wherever the centrosome may be located (pi. 14, figs. 40, 
46) at first, it ultimately comes to occupy the anterior pole of the 
nucleus (pi. 14, figs. 43, 44, 48, 54, c.). 
Changes in grov th of spermatozoon .— In the earlier stages of trans¬ 
formation and growth of the spermatozoon, the posterior or tail end 
is the only part which lengthens out, the centrosome, nucleus, and 
nebenkern being always found near the anterior end close to the 
enclosing cyst wall (pi. 14, figs. 40, 41, 46). As the spermatozoa 
increase in length, the cyst is transformed into a greatly elongated, 
hollow cylinder, with the elongated sperm cell arranged parallel 
within. 
When this transformation of the cyst is accomplished, the head end 
of the sperm cell begins to elongate, possibly from the increased lateral 
pressure (pi. 14, fig. 49). At the same time the nebenkern disappears 
