_Ecdysis._--The embryonic ectoderm of an insect consists of a layer of cells forming a continuous structure, the orifices in it--mouth, spiracles, anus and terminal portions of the genital ducts--being invaginations of the outer wall. This cellular layer is called the hypodermis; it is protected externally by a cuticle, a layer of matter it itself excretes, or in the excretion of which it plays, at any rate, an important part. The cuticle is a dead substance, and is composed in large part of chitin. The cuticle contrasts strongly in its nature with the hypodermis it protects. It is different in its details in different insects and in different stages of the life of the same insect. The "sclerites" that make up the skeleton of the insect (which skeleton, it should be remembered, is entirely external) are composed of this chitinous excretion. The growth of an insect is usually rapid, and as the cuticle does not share therein, it is from time to time cast off by moulting or ecdysis. Before a moult actually occurs the cuticle becomes separated from its connexion with the underlying hypodermis. Concomitant with this separation there is commencement of the formation of a new cuticle within the old one, so that when the latter is cast off the insect appears with a partly completed new cuticle. The new instar--or temporary form--is often very different from the old one, and this is the essential fact of metamorphosis. Metamorphosis is, from this point of view, the sum of the changes that take place under the cuticle of an insect between the ecdyses, which changes only become externally displayed when the cuticle is cast off. The hypodermis is the immediate agent in effecting the external changes. [Illustration: Adapted from Koerschelt and Herder and Lowne. FIG. 23.--Diagram showing position of imaginal buds in larva of fly. I., II., III., the three thoracic segments of the larva; 1, 2, 3, buds of the legs of the imago; h, bud of head-lobes; f, of feeler; e of eye; b, brain.]