A review of the methods available for the determination of the types of forces stabilizing structural proteins in animals.

F an animal is small and lives in an aqueous environment of constant composition, the necessary mechanical support and protection for its soft internal structures can be provided by skeletal materials of moderate stiffness and of moderate chemical stability. For larger animals, living in less constant aquatic environments or on dry land, it becomes necessary to provide more rigid support for the soft parts, and external membranes capable not only of preventing mechanical injury to soft tissues beneath, but also giving protection against osmotic changes and against desiccation, without themselves undergoing change as a result of the varying ionic content of the external medium, or as a result of humidity changes. The parasitic mode of life, or periodic exposure to drought in the case of aquatic or subaquatic organisms, or prolonged embryonic development, also necessitate the elaboration of protective and resistant surface membranes, cysts, and egg-shells. In many instances these requirements have been met by the development of membranes composed of proteins. The factors on which the mechanical and chemical properties of these depend can be ascertained by applying to their study certain methods which will be described here. The significance of the results obtained by these methods is only clear against the background of an elementary knowledge of the structure and properties of proteins.