NINE FIGURES
There is considerable evidence that suggests, universally so far as one can tell, that fibrous proteins such as muscle, collagen, and keratin are constructed of small corpuscular units. These units can be observed under appropriate experimental conditions that are, in general, foreign to the environmental conditions provided by the organism. Consequently, under physiological conditions all the structures chosen as examples are too insoluble to allow detection of monomers in equilibrium o r of precursor molecules. The mechanism of formation of fibrils therefore remains in question, and instructive information may be gained through studies of appropriate systems whose properties will allow deductions about mechanism. Thus knowledge of the mechanism responsible for the transformation of insulin into insulin fibrils may contribute to an understanding of the chemistry of insulin and of the possible mechanism of formation in. vivo of stable and ephemeral fibrils.
The formation of insulin fibrils may conveniently be produced by heating solutions of 2.0% insulin at p R 1.5-2.0. The resulting fibril suspension has been found, tlirough examinations of flow double refraction (Waugh, '44) or of electronmicrographs (Farrant and Mercer, ' 5 2 ) , to consist of a remarkably uniform population whose members are approxi-145 'SO), which breaks the original fibrils into segments. I t is quite apparent that the breaks produce ends having an un-