216 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
to secrete the scales ; they are the formative cells of the scales, — the 
“ Bildungszellen ” of Semper (’57). 
This is a most important stage, for it gives the best evidence yet pro- 
duced to prove the origin of the scale-producing cells. The fact that 
these cells contain each a vacuole, and that they are large and deeply 
staining, suggests that they may have arisen from the migratory leu- 
cocytes, some of which, as we have seen, are in like manner vacuolated, 
and stain deeply. There are, however, serious objections to this view. 
In the first place, the scale-producing cells are destined to secrete cu- 
ticula, like all ordinary hypodermis cells, and, so far as we know, this is 
not a usual function of mesenchymatous cells. In addition to this there 
are important considerations of a more direct nature, which point to the 
hypodermis, rather than to the mesenchyme as the source of these cells ; 
for at this stage some of the formative cells are still connected by their 
deep ends with the Grundmembran by means of protoplasmic prolonga- 
tions of their own cell bodies, just like the indifferent hypodermis cells. 
To my mind the evidence is perfectly satisfactory that the formative 
cells are simply modified hypodermal cells. 
In the next stage (Plate 2, Fig. 8) the scale-producing cell (sq.) has 
already grown outward as a blunt process, which bends distad, or to: 
wards the outer edge of the wing. The protoplasmic prolongations ( pre.) 
at the deep ends of the young formative cells have now nearly all dis- 
appeared, only a remnant of them being occasionally seen, as in the 
case of cell cl. frm. (Fig. 8). There is usually only a single vacuole 
in each of these young cells, but sometimes there are two, as in the 
case of the cell just referred to. 
Schiffer (’89, p. 643, Tafel XXX. Fig. 36) has described the condi- 
tions found in the pupal wings of Vanessa urtice about three days after 
pupation. The wing is in a slightly more advanced stage than the one 
shown in my Figure 8 (Plate 2). The formative cells are quite large, 
and each contains several small vacuoles (Secretblaischen); it is also 
worthy of remark that the formative cells now exhibit no traces of pro- 
toplasmic processes. My Figures 7 and 8 were drawn from pupe of 
Vanessa antiopa, which were kindly given me by Mr. Samuel Henshaw. 
The next older stage known to me is represented in Figure 28 (Plate 
5), and was drawn from a pupa of Danais plexippus. The formative 
cells (cl. frm.) have greatly increased in size, and the vacuoles, if they 
exist in this species, have entirely disappeared. The upward projections 
which are to form the scales (sg.) have grown outward to a much greater 
extent than in the stage last described. The hypodermis (/’drm.) is 
