DAVIS: SPERMATOGENESIS. 107 



The other part of the monosome assumes a similar shape, the change 

 being apparently brought about by the development, near the center, 

 of a thin area which finally breaks through forming a ring. A fur- 

 ther extension of this thinning process through one side of the ring 

 results in its interruption and the formation of a V-shaped boily. 

 This leads to the next stage (Figs. 143-145) in which both parts of the 

 monosome are distinctly V-shaped, although they can still be distin- 

 guished from each other since one has a rough contour while the other 

 is perfectly smooth. The two components of the monosome usually 

 lie close together, but may be separated a short distance, as in Figure 

 142, although in such cases they are always connected by linin fibers. 

 The components, as indicated in the figures, may be differently oriented 

 in relation to each other, in some cases lying side by side, in others 

 end to end. In the latter case the monosome often strikingly resembles 

 a tetrad (Fig. 145). The further changes consist in a shortening and 

 thickening of the two components, which usually fuse end to end, the 

 space between the arms of the Vs becoming gradually obliterated 

 (Figs. 146-148). Occasionally the components may lie side by side 

 (Figs. 149, 150), but apparently in such cases they are later connected 

 at only one end. Usually during the metaphase of the first matura- 

 tion division (Plate 6, Figs. 88, 91, 92) the monosome has the form of 

 a straight rod, — - the two components being indistinguishable, —  but 

 rarely it may be more or less curved. Probably such curved rods are 

 derived from forms like those shown in Figures 149 and 150. 



During the metaphase of the first division the monosome usually 

 lies nearer one spindle pole than the other (Plate 6, Figs. 88, 91, 92), 

 and, as in the preceding species, is easily distinguished by its more 

 ragged outline. During the anaphase (Fig. 93) the monosome does 

 not divide, but passes bodily to one pole. At this stage it shows a 

 distinct longitudinal split. In the secondary spermatocyte (Fig. 95) 

 the monosome is easily recognizable by its more compact structure. 

 Throughout this stage it shows a distinct longitudinal split. In the 

 second maturation division the monosome can be easily distinguished 

 by its relative size. Figure U (p. 106) is a polar view of the equatorial 

 plate in a cell which lacks the monosome, while Figure T is a similar 

 view of a cell containing this element. During the second maturation 

 division the monosome di^^des longitudinally and consequently one 

 half of each of the two components passes into each daughter cell. 



In Stenobothrus, as in the preceding species, there is a distinct 

 dimorphism of the spermatids, one half containing the monosome, 

 while the other half lacks this element. The monosome (Plate 5, 



