220 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
shrink, and has become coarsely granular (Plate 3, Fig. 12, and Plate 6, 
Fig. 36). This shrinking continues, so that the protoplasm comes to 
occupy only a portion of the cavity of the scale, as is shown in Figure 
37 (Plate 6). The protoplasm, indeed, begins to withdraw from the 
scales, and as it retreats from the free end of the scale leaves behind it 
little chitinous pillars (clm., Figs. 18, 31, 32, 37, and 41), which serve to 
bind the upper and lower surfaces of the scale together. This is shown 
in Figures 32 and 35 (Plate 6), which represent cross sections of the 
scales of Danais plexippus, and in Figure 17 (Plate 3), which shows a 
similarly sectioned scale of Callosamia promethea. 
It is well to mention here that Spuler (95, Tafel XXXVI. Fig. I) has 
already called attention to these chitinous pillars or “ Chitinbrücken,” 
as he calls them, in the scales of Galleria mellonella. 
The protoplasm continues its retraction until finally it is entirely 
withdrawn from the scales, and they become merely little flattened 
hollow chitinous sacs containing only air (sq., Plate 4, Fig. 18, and 
Plate 6, Fig. 38). The scales are now completely formed, but they still 
lack the pigment, this being introduced later. Owing to the fact that 
they are hollow, and contain only air, they diffract the light, and there- 
fore appear pure white, so that the whole wing is now in the “ white 
stage.” 
It will become evident from an inspection of Figures 31, 32, 34, 35, 
and 17, that the striations upon the upper surface of the scale are due 
to a series of parallel longitudinal ridges. The under surface (i.e. the 
one next the wing membrane) is usually smooth, or provided with few 
and poorly developed ridges. This fact was first pointed out by Burgess 
(80), who observed it in the scales of Danais plexippus. 
A still later stage than that shown in Figure 12 is illustrated by 
Figure 18 (Plate 4), which was drawn from a pupa of Danais plexippus 
about four days before the butterfly would have emerged. The wings at 
this stage are slightly ochre-yellow in color, for the protoplasm has 
entirely disappeared from the scales and the pigment is just beginning to 
form. But by far the most remarkable change is to be noticed in the 
nuclei of the formative cells (cl. frm.). (Compare Fig. 12 with Fig. 
18, Plate 4.) The chromatin has shrunk into a solid ball of deeply 
staining substance, and lies in the centre of the clear vesicular nuclear 
space. In many of the formative cells, strange to relate, the nucleus 
begins to divide amitotically, as is to be inferred from the conditions 
shown in Figures 19-22, which I believe to represent successive stages 
in the process of nuclear division. As a result of this process, we often 
