Life: Its Nature and Origin 



49 



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Fig. 21. Collagen, a protein of muscle, cartilage, and bone, (a) Normal 

 collagen fibrils, (b) Collagen dispersed as thin filaments by immersion in dilute 

 acetic acid, (c) Collagen fibrils formed spontaneously from filaments such 

 as those in (b) when 1 per cent of sodium chloride has been added to 

 the suspension of dispersed filaments; the resulting long fibers are identical 

 in appearance but not quite in structure to the fibrils in (a) which were dis- 

 persed to form the filaments in (b). (d) Collagen fibers in human leg bone 

 after removal of calcium. These electron microscope photographs represent a 

 magnification of about 30,000 X. (Photographs of (a), (b), and (c) courtesy 

 of F. O. Schmitt, Proc. Am. Philos. Soc. 100:478, 1956; photograph of (d) 

 courtesy of R. A. Robinson. ) 



