DAVIS: SPERMATOGENESIS. 71 
chromosomes shorten and thicken, the asters and centrosomes increase 
rapidly in size and become connected by a well defined central spindle. 
At the same time the nuclear membrane gradually disappears, first 
disintegrating on the side adjacent to the asters. Figure 11 shows 
a somewhat later stage, in which the nuclear membrane has entirely 
disappeared and the spindle is moving in among the chromosomes, 
which are connected at one end with the astral rays. At a little later 
stage (Fig. 13) the spindle is much larger and is composed of two 
types of fibers, the coarser mantle fibers attached to the chromosomes 
and the finer central fibers. ‘lhe chromosomes are now arranged in 
a single plane around the periphery of the spindle and are attached 
at one end to the mantle fibers. At this stage the centrosomes reach 
their greatest development and can be distinctly seen as large deeply 
staining bodies at the poles of the spindle. ‘The astral rays are now 
few and indistinct. At no time during mitosis is there any sign of a 
centrosphere around the centrosome. 
The chromosomes now all divide longitudinally and pass to the 
poles of the spindle, as in ordinary mitotic division (Fig. 14). 
Cross sections of the spindle in the metaphase (Figs. B, C, and 
Plate 1, Fig. 12) show in favorable cases twenty-three rod-shaped 
chromosomes arranged in a ring around the periphery of the spindle. 
These chromosomes vary greatly in size and a careful study shows 
that they form a nicely graded series from the largest to the smallest. 
It is also evident that, as has been shown by a number of recent in- 
vestigators in other insects, the gradations in volume are not between 
single chromosomes, but between pairs of chromosomes. In Figures 
B and C I have attempted to indicate the members of each pair, 
although on account of fore-shortening and the slight difference in 
volume in some cases no pretence is made to complete accuracy. 
There are three large pairs 1, 2 and 3 (Figs. B,C). The next chromo- 
some, 4, is without a corresponding mate and is therefore the mon- 
osome. Following this the pairs 5, 6, 7, 8, 9, 10 form a series 
constantly diminishing in size. There is a marked break in size 
between 10 and the larger of the two smallest pairs, 11 and 12. 
(The different pairs are much more clearly marked in Stenobothrus, 
where there is a greater difference in size.) There are thus in the 
spermatogonia twenty-two autosomes and one monosome. 
Figure 14 (Plate 1) shows the anaphase, Figure 15 the telophase of 
the spermatogonial division. In the telophase as the chromosomes 
begin to break down the projecting end of each is usually enclosed in 
a distinct vesicle. It is, however, probable that these vesicles are 
