Six or seven days after the last larval moult the chitinous wall is formed, the wing remaining transparent. It grows rapidly and its lower edge extends near the legs. It is now much crumpled on the edge, owing to its rapid growth within the limits of its own segment. Partly from being somewhat retracted, and partly owing to the irregularity of its surface, the wing gradually separates from its envelope, and the cavity of invagination (Fig. 145, _c_) becomes more like a distinct or real space. The outer opening of the alary sac enlarges quite plainly, though without reaching the level of the edge of the wing. This condition of things does not still exactly explain how the wing passes to the outside of the body. Gonin compares these conditions to those exhibited by a series of sections of the larva, made forty-eight hours later, on a caterpillar which had just spun its girdle of silk. At this time the wings have become entirely external, but, says Gonin, we do not see the why or the how. The partition of the sac has disappeared, and with it the cavity and the leaf of the envelope. It appears probable that the partition has been destroyed, because the space between the two teguments is strewn with numerous bits, many of which adhere to the chitinous integument, while others are scattered along the edges of the wings, in their folds, or between the wings and the wall of the thorax. Another series of sections showed that the exit of the fore wings had been accomplished, while the hinder pair was undergoing the process of eversion. In this case the partition showed signs of degeneration: deformation of the nuclei, indistinct cellular limits, pigmentation, granular leucocytes, and fatty globules. After the destruction of the partition, what remains of the layer of the envelope is destined to make a part of the thoracic wall and undergoes for this purpose a superficial desquamation. The layer of flattened cells is removed and replaced by a firmer epithelium like that covering the other regions. It is this renewed hypodermis which conceals the wing within, serves to separate it from the cavity of the body, and gives the illusion of a complete change in its situation. Other changes occur, all forming a complete regeneration, but which does not accord with the description of Van Rees for the Muscidæ. Finally, Gonin concludes that the débris scattered about the wing comes from the two layers of the partition of the sac, from the flattened hypodermis of the renewed envelope, from the chitinous cuticle of the wing, and from the inner surface of the chitinous integument. He thinks that the metamorphosis of Pieris is intermediate between the two types of Corethra and of Musca, established by Weismann, as follows: =Corethra.=—The wing is formed in a simple depression of the hypodermic wall. No destruction.