[Illustration: Fig. 174—Transverse section of part of the newly disclosed chrysalis of _Pieris brassicae_, showing the position and structure of the wings, hanging from one side of the body. _aa_, Anterior wing; _ap_, posterior wing; _e_, _e_, peritracheal spaces; _t_, _t_, tracheae. (After Gonin.)] The formation of the scales of the wing commences very early—apparently soon after the casting of the larval skin—though the completion of the scales and their pigmentation is delayed to a late period of the pupal life. The scales are formed by special cells of the hypodermis that are placed deeper in the interior of the wing than the other hypodermal cells. Each scale is formed by one cell, and protrudes through the overlying hypodermis; the membrane into which the scales are inserted is a subsequently developed structure, and the beautiful {334}articulation of the scale with the wing takes place by a division of the stalk of the scale where it is encompassed by the membrane. Semper was not able to show that the scale-forming cells are certainly hypodermal cells, but this has since been demonstrated by Schäffer, who also shows that each of the cells contains an excretory vesicle. [Illustration: Fig. 175.—Early condition of scales and nervures. (After Semper.) A, Section of portion of wing of pupa of _Sphinx pinastri_; _a_, basal membrane with trachea beneath it; _c_, scale-forming cell; _d_, early state of a scale; _e_, _e_, more advanced stages; _f_, hypodermal cells. B, part of a cellular cylinder that excretes the nervure [or more probably the rib or "Rippe" of Schäffer; cf. Fig. 170, B]; _b_, epithelial [hypodermal] cells; _a_, central string [supposed by Semper to be a nerve].]