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 

 t\'pes of fibers, the coarser mantle fibers attached to the chromosomes 

 and the finer central fibers. The 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 di^ide 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 chfference 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 spermatogonia! 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 



