THE FINE STRUCTURE OF BIOLOGICAL SYSTEMS

Further Studies on the Development of Cysticercus fasciolaris and Cysticercus pisiformis, with special reference to the Growth and Sclerotization of the Rostellar Hooks

The previous observations of the author and others on the development ot Cysticercus fasciolaris and C. pisiformis have been confirmed and amplified by experimental studies, and the developmental processes of the two forms are compared. While definite information has been obtained on the age of the various developmental stages, there is considerable variation due to the subtleties of the host-parasite relationship.

The study of the development of the rostellar hooks of taeniids has been extended by the use of differential staining, histochemical tests and polarized light, and new facts, such as the chemical and optical differences existing between cuticle and hooklets and between blade and base of hooks, have been brought to light.

Shape in relation to fine structure in the bristles of Drosophila melanogaster

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 bristle 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.