[Plate 16]The bristles of Drosophila melanogaster have provided m aterial for a stu d y of th e relation betw een th e shape of a specialized ty p e of cell an d th e o rientation of th e chitin molecules of th e cell wall, as revealed by optical an d m echanical properties. Close correspondence betw een shape an d o rientation has been found, b o th in norm al an d m u ta n t bristles. Certain of th e m echanical properties of th e wall of th e ad u lt bristle can be studied directly, an d som ething of those of th e im m ature w all can be inferred from th e behaviour of th e bristle during develop m ent. There is reason to believe th a t th e new ly form ed wall is plastic an d th a t it possesses a definite yield value. The bristle m ay be regarded as a hollow object blown in a plastic, aniso tropic m edium b y th e pressure of cytoplasm form ation in th e trichogen. I t seems reasonable to suppose th a t th e shape of th e norm al bristle is intim ately related to th e grow th in length of th e chitin chains of th e cell wall oriented parallel to th e long axis of th e cell. Indeed th e long axis appears to be such because th e oriented chains grow in its direction. Our stu d y of m u ta n t as well as norm al bristles has enabled us to m ake a te n ta tiv e analysis of th e factors concerned in th e ir grow th, an d to suggest w hat m ay be th e m ode of action of those genes w hich m odify th e shape of th e bristle. W e are led to an in te rp re tatio n of th e shape of th e bristle in term s of th e properties of th e substances secreted b y th e trichogen (wall-substance an d cytoplasm ) an d of th e tim e relations of th e ir synthesis.
Summary
1. Present‐day interest in the study of fine structure may be traced back to the work of the botanist, Carl Nägeli, to whom the conception of the micell is due.
2. The continuation of Nageli's optical studies of biological structures, the development of X‐ray analysis, and the investigation of the properties of long chain high polymers, have led to the extension and modification of Nägeli's ideas.
3. Two main types of molecular aggregate are of particular importance in biological systems—linear (or fibrous) and laminar structures. These may be classified as subcellular, extracellular and supracellular.
4. The subcellular structures which have been examined include: protein fibres from the sap of virus‐infected plants; chromosomes; asters and spindles; and contractile fibrils (muscle fibrils, myonemes, pseudopodia and cilia). These all appear to be composed of chain molecules arranged approximately parallel to the long axis of the structure in question.
Laminar subcellular structures include the surface membranes of animal cells (nerve fibres, echinoderm eggs, red blood corpuscles) and the outer portion of the rods and cones of vertebrate retinae. These are composed apparently of alternating layers of protein and lipoid molecules.
5. Extracellular fibrous structures are represented by elastoidin, collagen, elastin and chitin; laminar structures by the keratinous egg cases of selachians, the cellulose test of tunicates and chitinous integuments in general. The relation of these to other natural and synthetic high polymers is discussed.
6. The category of supracellular Structures, that is, of macroscopic fibrous or laminar structures of multicellular origin includes (a) hair and muscles, and (b) enamel and bone.
7. It is clear that a close parallel in structure and behaviour exists between biological materials and high polymer substances. The morphological implications of this parallel are discussed and the importance of molecular morphology for the biologist is considered.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.