332 FINE-STRUCTURE OF PROTOPLASMIC DERIVATIVES III 



silk-gland. A finer structure is revealed both by the sericin layer and 

 by the fibroin threads (Ohara, 1933 a). On the outside is a very 

 weakly birefringent, almost amorphous membrane of sericin, under 

 which comes a strongly birefringent layer of a fibroid texture. The 

 sericin layer is separated from the fibroin threads by an isotropic 

 lamella. It is here that the sericin becomes detached from the fibroin 

 when the silk is degummed in a dilute soap solution. Two degummed 

 silk threads are then formed from every cocoon filament or raw silk 

 thread. The difference between raw silk and ordinary degummed silk 

 is therefore that the former is still surrounded by the sericin cortex, 

 though admittedly this often suffers considerable mechanical damage. 



The fibroin filaments, which are now to form our main topic, have, 

 according to Ohara, three zones which are optically distinguishable, 

 i.e., a central zone of fibrous texture, a cortical layer around this of 

 fibroid texture and, finally, at the outside a skin layer. This is only 

 slightly anisotropic, yet its texture is apparently slightly fibroid. It is 

 interesting to note that here the prevailing direction of orientation 

 — i.e., deviation of the optically positive submicroscopic fibroin 

 rodlets — is not tangential, as in the sericin layer or in cellulose fibres, 

 but radial (see Fig. i68b). It seems that in the process of degumming, 

 the character of the scattering in the coating layer changes from radial 

 to tangential, for, after the hot water treatment, the large axis of the 

 index ellipse lies tangentially. The scattering of the fibroid texture of 

 the cortical layer is likewise radial. Thus in a cross-section through 

 the cocoon threads, sericin and fibroin are easily distinguishable by 

 their different optical behaviour in a polarizing microscope in which 

 a selenite test plate has been inserted, in that the sericin wrapping 

 produces a negative, and the fibroin cortex a positive spherite cross 

 (cf. Fig. 66a, b, p. 96). In the cross-section the central zone appears 

 to be isotropic. 



The zoning of the fibroin filament is brought out clearly by dichroic 

 gold and silver staining. In conformity with its fibrous texture, the 

 central zone exhibits pronounced dichroism; in the cortical zone, on 

 the other hand, with its far inferior orientation, the coloration is not 

 dichroic, and in the coating layer there is none at all. According to 

 Ohara (1933 a), this is how the fibroin filament laminates as a result 

 of coagulation: The coating layer is the first to coagulate on leaving 

 the silk-gland, before there is any opportunity for an ordered sub- 



