76 BRITISH LEPIDOPTEEA. 



slightly in size, to project a little above the level of the ordinary hypo- 

 dermic 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. 



It may be well to add here, that Jackson has observed (Trans. Linn. 

 Soc. Land., 2nd series, v., p. 166) a cuticular secretion apparently 

 fluid, formed by the hypodermis just before the appearance of these 

 scales. He says that, at this period, the bypodermic cells themselves 

 readily separate from the pupal cuticle, and that on their surface is a 

 darkly staining layer, looking like a new cuticle, which he believes to 

 be a coagulable fluid, for, in the next stage, it is increased in amount, 

 and, in some of the sections, the growing scales may be seen imbedded 

 in it. He also notes that it appears at a later period to be drawn out 

 into irregular bands and filaments by the separation of the pupal 

 cuticle from the hypodermis. 



In the next stage the scale-producing cell fsq. ) 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 (PL i., 

 fig. 12). 



The pupal wing of Aglais 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 

 Anosia archippus) 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 cor- 

 responding in position with a row of formative cells, and each furrow 

 with the interval within two adjacent rows. As a consequence, the 

 scales always project from the tops of these ridges. The " grund- 

 membran " does not partake in the folding, and the deep processes of 

 the hypodermal cells, that once extended to this membrane, have now 

 disappeared (PL L, fig. 13). 



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

 the inner cuticular membrane, which previously lay almost in contact 

 with the hypodermal cells, has been pushed outward by the develop- 

 ment of the scales (vide, PL L, fig. 7). The growth of a single scale at 

 this period, separating the cuticular covering of the pupa, is shown in 

 PL L, fig. 6, where the scale sq. is seen in connection with the forma- 

 tive cell (ci.fnn.) of the scales (vide, PL L, fig. 7). The protoplasmic 

 processes which joined the hypodermis to the " grundmembran " (mbr. 

 pr.) have disappeared, the latter being now nothing more than a simple 

 homogeneous structure, with the appearance of a structureless mem- 

 brane lying below the hypodermis (PL L, fig. 7). At this, and, still 

 better, at a little later, stage of development, it is observed that the 

 body of the large formative cells lies below the level of the ordinary 



