WING, WING-SCALES, AND THEIR PIGMENTS IN BUTTERFLIES AND MOTHS. 133 



dermis cells, and, most remarkable of all, to acquire each a vacuole." 

 These modified cells are destined to give rise to the scales. They are 

 the formative cells of the scales, the " Bildungzellen " of Semper. 

 The evidence at present available tends to show that these scale-pro- 

 ducing cells are hypodermal, and not mesenchymatous cells, that they 

 are, in fact, modified hypodermal cells. 



In the next stage the scale-producing cell (sq. j has already grown 

 outward as a blunt process, which bends distad or towards the outer 

 edge of the wing. The protoplasmic prolongations at the deep ends 

 of the formative cells have nearly all disappeared. There is usually 

 only one vacuole, occasionally there are two, in each of these cells 

 (PI. iii., fig. 12). 



The pupal wing of Ai/lais urticae, three days after pupation, shows 

 a slight advance in development on the above. The formative cells 

 are quite large, and each contains several small vacuoles ; they no 

 longer exhibit any trace of protoplasmic processes. 



At a slightly more developed stage (the pupa examined is that of 

 A. a)-cldppm) the formative cells have greatly increased in size, and 

 the vacuoles have entirely disappeared. The upward projections, 

 which are to form the scales, have grown outward to a much greater 

 extent than in the stage last described. The hypodermis is thrown 

 into a regular series of transverse ridges (across the nervures), each 

 ridge corresponding in position with a row of formative cells, and 

 each furrow wnth the interval between two adjacent rows. As a con- 

 sequence, the scales always project from the tops of these ridges. The 

 " grundmembran " does not partake in the folding, and the deep pro- 

 cesses of the hypodermal cells, that once extended to this membrane, 

 have now disappeared (PI. iii., fig. 13). 



About eight days before the emergence of the imago of A. archippus, 

 the inner cuticular membrane, which previously lay almost in con- 

 tact with the hypodermal cells, has been pushed outward by the 

 development of the scales {vide., PI. iii., fig. 7). The growth of a 

 single scale at this period, separating the cuticular covering of the 

 pupa, is shown in PI. iii., fig. 6, where the scale s(j. is seen in con- 

 nection with the formative cell {cl.frm.) of the scales {vide., PI. iii., 

 fig. 7). The protoplasmic processes which joined the hypodermis to 

 the "grundmembran" (/»ir. /)/•.) have disappeared, the latter being now 

 nothing more than a simple homogeneous structure, with the appear- 

 ance of a structureless membrane lying below the hypodermis (PI. iii., 

 fig. 7). At this, and, still better, at a little later stage of develop- 

 ment, it is observed that the body of the large formative cells lies 

 below the level of the ordinary cells, and sends a protoplasmic process 

 upward to form the scale. This is well exhibited in PI. iii., fig. 6. 

 The scale at this stage is a minute flattened chitinous bag, filled with 

 protoplasm, and whilst the scales remain full of protoplasm, they 

 appear as transparent as glass, but when the protoplasm shrinks out 

 of them they become whitish. 



The hypodermal cells, although no longer separated by well-defined 

 cell walls, are still well marked cut by the peculiar arrangement of the 

 finely granular contents of the cells. The hypodermis, too, has now 

 begun to secrete the chitinous cuticula of the wing membrane ; but it 

 is as yet very thin, becoming much thicker as the wings develop. 

 Each of the hypodermal cells, at this stage, gives rise to a new pro- 



