534 FINE-STRUCTURE OF PROTOPLASMIC DERIVATIVES III 



similar to that found in cellulose fibres, when the central portion, 

 expanding powerfully, is pressed through weakened spots of the skin 

 layer (Fig. 169). Furthermore, the central zone can be split up bv 

 bromine lye (hypobromite) into fibrillae, which then disintegrate into 

 short bundles of fibrillae, as in cellulose fibres. These facts are im- 

 portant, in that they imply, contrary to Ludtke's statement (1936) 

 about cellulose fibres, that beading and hydrolytic disintegration 

 perpendicular to the fibre axis do not depend upon any partition across 

 the fibre ; for there can be no question of the formation of any such 

 hypothetical segmentation during the generation of the silk thread. 



Just as with other fibrous structures, a system of submicroscopic 

 rodlets is to be expected; and, in fact, Ohara (1933a) found rodlet 

 double refraction in silk. However, as the minimum of the curves he 

 has published (n ^ i-47) does not agree with the average refractive 

 index of silk fibroin, his measurements do not seem altogether 

 reliable. Hegetschweiler (1948) finds (n,,)^ = 1.5960 and (nj^ 

 = 1.5454 yielding the double refraction An = 0.0506 for silk. These 

 figures give an average of (n^, -f 2nJ/3 = 1-565, which is quite in- 

 compatible with n = 1.47 mentioned above. For this reason, Heget- 

 schweiler (1950) repeated the imbibition experiments of Ohara and 

 found that liquids which do not swell silk fibroin cannot penetrate 

 and, therefore, do not change its birefringence. Since the cross- 

 section of the fibroin thread is triangular (Fig. 168 a, p. 331), so that 

 the thickness corresponding to a retardation of light F observed can 

 only be measured after rotation of the thread through 90°, and as the 

 swelling in aqueous solutions is considerable, it is very difficult to 

 obtain reliable figures reflecting small changes of the birefringence of 

 silk by the formula Zl n = F/d. If all the necessary precautions are 

 taken and numerous measurements made in the same liquid in order 

 to obtain reliable average values, it can be proved that silk fibroin is 

 not a mixed Wiener body. 



This optical finding is borne out by electron microscope investi- 

 gations (Hegetschweiler, 1950). Unlike native cellulose, silk fibroin 

 does not consist of individual microfibrils. There is a distinct fibrillar 

 texture, but the diameter of the visible strands depends on the method 

 of preparation. The same fibroin threads show rather coarse (o.i ju) 

 or very fine (0.0 1 /u) strands, or both types together with intermediate 

 grades, depending on the way in which they have been hit during 



