Nymphs of Rhodnius decapitated 24 hr. after feeding can be induced to moult by implanting into the abdomen the dorsal region of the protocerebrum removed from other nymphs during the critical period. Implantation of other parts of the brain, of the corpus allatum, and of fat body from the same insects did not cause moulting. The presence of large nerve cells with fuchsinophil inclusions discovered by Hanström in this region of the brain has been confirmed. The histological changes in the epidermis of the abdomen and the distribution of mitoses at an imaginal moult and at a nymphal moult have been compared. During a nymphal moult mitoses occur all over the tergites; during an imaginal moult they are largely confined to the intersegmental membranes and the periphery of the abdomen, and there is a more extensive breakdown of existing cells. If 5th stage nymphs in the course of moulting to become adults receive "inhibitory hormone" from young nymphs, they may be caused to "switch over" to nymphal development. Such a "switch over" soon becomes impossible for the most specialized structures of the adult; other structures follow in turn; but the general cuticle of the tergites may still be influenced up to a short time before it is due to be laid down. The various faculties of a given cell can become determined to some extent independently of one another. Isolated fragments of cuticle and epidermis from Rhodnius adults may be induced to moult, more than once, by transplantation to young moulting nymphs. Decapitated Cimex adults may be caused to moult again if they receive blood from moulting Rhodnius nymphs; but they lay down a normal cuticle with bristles only if they have become adult very recently. Decapitated Rhodnius adults may be caused to moult again if they receive blood from two moulting 5th stage nymphs. They lay down a cuticle of normal adult type even when they have been adults for several months. The old skin is digested up to the level of the exocuticle. If such moulting adults are provided with inhibitory hormone from the corpus allatum of young nymphs, they show a partial reversion to nymphal characters when they moult. This change probably does not extend to the most specialized imaginal structures; but the pigmentation and the structure of the general cuticle, and of the bristles it carries, may become partially nymphal again. The "determination" of imaginal or nymphal characters thus takes place at different times in different organs. And for some characters, at least, such determination is not irrevocable. In the light of these results a new hypothesis is put forward to explain the action of the inhibitory hormone in controlling metamorphosis. This work was begun during a stay of three months in Berlin-Dahlem as the guest of the Kaiser-Wilhelm Gesellschaft, to whom my thanks are due. Throughout this stay I was indebted to Prof. A. Kühn and his colleagues at the Kaiser-Wilhelm Institut für Biologie for every assistance and for much stimulating discussion.
