AND OF HYBRID PIGEONS. 
31 
bisection of the central filament has been accomplished, and the 
nucleus displays itself as an enormously long sinuous head 
which may measure twice the length of the adult spermatozoon 
head. A dense protoplasmic mass encases it and extends 
backwards along the axial filament (Fig. 25). 
A shrinking or condensation of the nucleus follows, and as 
this progresses, the double spiral of chromatin shortens and 
widens until it attains to the characteristic appearance shown 
in Fig. 26. From this point, through the further condensation 
of the head, the openings of the spirals are crowded closer to¬ 
gether and the exact relationship of the chromatin of the two 
filaments can no longer be determined. Whether or not the 
double spiral structure is finally lost could not be definitely 
determined, but if it persists, it is considerably obscured and the 
general aspect of the interior of the head becomes that of a 
series of vesicles (Fig. 27). That is, the chromatin seems to be 
arranged like the links of a chain, each link of which incloses 
a clear area which in some preparations appears highly refrac¬ 
tive. A remarkable fact is that the number of these links or 
vesicles is apparently the same as the reduced number of uni¬ 
valent chromosomes should be, namely, eight. Not infre¬ 
quently there were only six or seven of the vesicles, but in such 
cases one or two were much longer than the ordinary ones and 
hence possibly equivalent to two. It will be recalled that in the 
spermatid only four chromosomes were to be seen, but that 
they were of the bivalent type or equivalent to eight ordinary 
chromosomes. There is no positive evidence that the vesicles 
in the head of the spermatozoon correspond to individual 
chromosomes, but the coincidence in number is at least very 
suggestive and it would not be surprising if later the fact devel¬ 
ops that after the entrance of the spermatozoon into the egg, 
the vesicles resolve themselves into eight distinct chromosomes. 
Shortly before its development is completed, the head bends 
or curls backward tov/ard the tail and lies in a dense cyto¬ 
plasmic mass, thus passing apparently into a short resting or 
finishing stage. The typical shape of the head in this condition 
is that of a horseshoe (Fig. 26) but frequently it may be twisted 
into a series of loops, especially in the earlier stages of conden¬ 
sation, in much the same fashion as a string will curl if one end 
