The bristles of Drosophila melanogaster have provided material for a study of the relation between the shape of a specialized type of cell and the orientation of the chitin molecules of the cell wall, as revealed by optical and mechanical properties. Close correspondence between shape and orientation has been found, both in normal and mutant bristles. Certain of the mechanical properties of the wall of the adult bristle can be studied directly, and something of those of the immature wall can be inferred from the behaviour of the bristle during development. There is reason to believe that the newly formed wall is plastic and that it possesses a definite yield value. The bristlo may be regarded as a hollow object blown in a plastic, anisotropic medium by the pressure of cytoplasm formation in the trichogen. It seems reasonable to suppose that the shape of the normal bristle is intimately related to the growth in length of the chitin chains of the cell wall oriented parallel to the long axis of the cell. Indeed the long axis appears to be such because the oriented chains grow in its direction. Our study of mutant as well as normal bristles has enabled us to make a tentative analysis of the factors concerned in their growth, and to suggest what may be the mode of action of those genes which modify the shape of the bristle. We are led to an interpretation of the shape of the bristle in terms of the properties of the substances secreted by the trichogen (wall-substance and cytoplasm) and of the time relations of their synthesis.