2 PROTEINS 333 



microscopic structure to be formed. A little later, the cortical layer, 

 the jfibroin thread molecules of which are already to some extent 

 orientated, coagulates. The fibroin mass in the central zone remains 

 plastic for a longer period and the chain molecules of the silk fibroin 

 are all able to orientate with parallel axes before they combine to form 



•fs!e*-.«=5S«.-., 



^^"^ ik 



7> 



- -c? 



r 



v< 



a) b) 



Fig. 169. Fine-structure of silk (from Ohara, 1933b). a) Beading with Ca(N03)2 solution; 



b) fibrillar formation with hypobromite. 



a micellar frame. As rayon filaments often display a similar structure 

 (Preston, 1933), Ohara's hypothesis is attractive, but it should be 

 pointed out that, unlike natural silk, the cortical part of viscose is 

 submicroscopically better orientated than the central part of the 

 filament ("skin effect" according to Preston, 1933). In rayon, the 

 stretching process brings about an orientation of the peripheral region, 

 whereas the thread molecules of the still uncongealed mass in the 

 centre of the filament are not effectively held by the orientating forces, 

 owing to their mobility. Hence the optical conditions prevailing in 

 natural silk which conflict with this interpretation must be explained 

 in some other way. 



Since, like vegetable bast fibres, the silk fibroin filaments possess 

 a central portion of a fibrous texture and a skin with a pronounced 

 deviation of the microfibrils from the direction of the fibre axis, their 

 swelling and hydrolysis phenomena are similar to the cellulose walls 

 ■of fibre cells. Thus Ohara (1933b) finds a beaded appearance in silk 



