DAVIS: SPERMATOGENESIS. 95 
61). Apparently the centrosomes are already some distance apart 
when first distinguishable, and continue to migrate around the nucleus 
until they come to lie opposite each other. ‘he nuclear membrane 
then gradually breaks down, first disappearing in the vicinity of 
the asters. As the nuclear membrane disappears the astral rays extend 
into the nucleus and become attached to the chromosomes. Figure 
62 shows a somewhat later stage, after the spindle is fully formed, but 
before the chromosomes are drawn into the equatorial plane. ‘The 
autosomes have now become homogeneous and have perfectly smooth 
contours, but they retain, in general, the characteristic forms seen in 
the earlier stages. Figure 63 shows the metaphase of the first matura- 
tion division. As the chromosomes become drawn into the equatorial 
plane, the rod-shaped forms are arranged with their long axes parallel 
to the axis of the spindle, the mantle fibers being attached at each end 
(Plate 7, Fig. 164). ‘Thus the spindle fibers from one pole are attached 
to one of the univalent components of the bivalent autosomes, those 
from the other pole to the other univalent component. ‘lhe cross- 
shaped autosomes become so arranged that one pair of arms lies along 
the spindle while the arms of the other pair project away from the axis 
of the spindle and, at the same time, make a considerable angle with 
each other, so that in a polar view they appear V-shaped, the apex of 
the V being directed toward the spindle. In these autosomes it is 
impossible to determine whether the plane of union of the univalent 
components is at right angles to the axis of the spindle or parallel to it, 
but from analogy with the other types it is probably at right angles. 
In the case of the closed rings and crossed loops the evidence is more 
satisfactory. ‘hese chromosomes become so arranged on the spindle 
that the free end of each univalent component is attached to mantle 
fibers from the opposite pole of the spindle, while the apex, which 
represents the point of union of these components, projects away from 
the spindle (compare Figs. 179, 180). 
In the metaphase of the first maturation division there are always 
twelve chromosomes, eleven of them being bivalent autosomes, which 
vary greatly in size. Figure N is a polar view of the equatorial plate 
in two adjoining cells and shows that in general the autosomes have 
the same size relations as the autosome pairs of the spermatogonia, 
although in the spermatocytes the relative size of the chromosomes, 
on account of their irregular form, is not as clearly marked. Inasmuch 
as the univalent components of each bivalent autosome are of equal 
size, there seems to be no good reason for doubting that each bivalent 
autosome is formed by the union of two homologous autosomes of 
the spermatogonia. 
