544 FINE-STRUCTURE OF PROTOPLASMIC DERIVATIVES III 



very significant, as it shows how readily the frame of proteins can be 

 destroyed and built up again. Seeing that hot water suffices to initiate 

 this process in keratin, it is not difficult to imagine how, in the far 

 more labile cytoplasm, the protein thread molecules are constantly 

 forming new combinations and side bonds, while others are con- 

 tinually being broken down, so that a definite molecular framework 

 is always in existence, despite the apparent liquid state of the material. 



Fine-structure of finger nails. Finger nails are built up of submicro- 

 scopic fibrillae. X-ray analysis shows that the keratin fibril lae run, not 

 parallel, but perpendicular to the longitudinal axis of the nail (Derk- 

 SEN, Heringa, and Weidinger, 1957). As in the elongation growth 

 of the plant cell wall, therefore, the micelles are orientated perpen- 

 dicular to the direction of growth. The alignment of the micellar 

 strands, therefore, is not a passive process due to the forces of growth 

 pushing the nail forward; there are special formative forces at work, 

 building up submicroscopic textures with due regard to their future 

 functions. 



By maceration with NaoS a thin, 100 A thick membrane can be 

 detached from the surface layers of human skin and finger nails. It is 

 compared with the epicuticle of wool (Lagermalm, Philip and 

 Lindberg, 195 i). 



Fsather keratin. Not all horny substances are naturally in the state 

 of a-keratin. Instead of the fibre period for mammalian hair, viz., 

 5.06 A in the direction of the primary chain, that of quills in the un- 

 extended state is 3.1 A (Astbury and Marwick, 1932). By elongation 

 it can be increased continuously and reversibly to 3.3 A but, if 

 subjected to further elongation, the quill breaks. From this fact it 

 may be concluded that the polypeptide chains in quill keratin are 

 stretched approximately in the same way as in elongated hairs or in 

 silk fibroin. The fact that the length of the members of the primary 

 chains is neither 3.38 A nor 3.5 A is said to be due to slight corrugation 

 (so-called "primary folding") of the polypeptide chains in the feather 

 keratin, owing to a certain interaction of the side chains. This slight 

 primary folding is also supposed to be responsible for the shortness, 

 as compared with silk fibroin, of the amino acid residues of ^-keratin. 

 The far sharper kinks in the a-keratin chains are distinguished from 

 this slight corrugation as "secondary folding". Thus the sclero- 

 protein of quills is a modification of keratin in which there is no 



