permatocyte
division. The element _x_ is very often concealed by the polar
aggregation of chromatin, but it is sometimes as conspicuous as in
figures 131 and 132. The spermatocytes of the second order go into a
complete resting stage before they are completely separated, and one of
a pair shows the element _x_, while it is lacking in the other (fig.
133). At the close of the resting stage the chromosomes appear as 11
pairs of rods of considerable length, which gradually shorten and
thicken and usually bend at the center, forming U's or V's (figs.
134-138). In one stage these double U's look much like tetrads (fig.
138). The rods straighten again as they shorten still more (fig. 139),
become more closely approximated, and finally form dumbbells, as in
figure 141.

The element _x_ is, of course, present in only one-half of these nuclei.
In the equatorial plate, figure 142, it is absent; in figure 143 it is
present, but can not be distinguished from the other chromosomes, while
in figure 144 it is rendered conspicuous by its spherical form and
isolated position. In only a few cases has it been possible to
distinguish _x_ in the spindle. Figures 146 and 147 show two of these
cases where this element is clearly double and of different form from
the other chromosomes. It is probable that it divides and so goes into
one-half of all of the spermatids, as in McClung's typical cases of the
accessory chromosome. Figure 145 shows the usual appearance of the other
chromosomes in metaphase. The two spermatids of a pair are always alike
so far as any evidence of the presence of the element _x_ is concerned
(fig. 148). Figure 149 is an exceptional case, where one chromatin
element (possibly _x_) has evidently divided late and been left out in
the cytoplasm; a smaller chromatin granule is also present in the
cytoplasm of each spermatid. All of the spermatids, as in
_Stenopelmatus_, develop a deeply-staining body, which, however, in this
case is usually centrally located and often appears double (figs.
150-152).

The spindle-remains (_Spindelreste_) forms a very conspicuous body at
one side of the nucleus in the spermatids, and occasionally a mass of
chromatin, probably due to imperfect mitosis, is found near the
spindle-substance (fig. 150). The mass of spindle-substance at first
appears structureless, but soon assumes the condition shown in figures
150 to 152. In one individual many of the spermatids had two balls of
spindle-materia