30,8 



FINE-STRUCTURE OF PROTOPLASMIC DERIVATIVES 



III 



In nature, therefore, there are very probably crossed micro- 

 structural systems, the fibrillar structural elements of which interweave 

 orthogonally or at other angles. The establishment of this fact may 

 assist very materially in clarifying the submicroscopic texture of ex- 

 ceedingly thin membranes. 



Fig. 150. Affine growth (from Rosin, 1946). a may grow larger progressively in pro- 

 portion to h, or towards one side {c,d,e) ; the crossed system thereby remaining orthogonal. 



It is satisfactory that these speculations can be substantiated by 

 electron microscopy. In primary cell walls (Frey-Wyssling, Muhle- 

 THALER and Wyckoff, 1948) and in the Tunicata mantle (Frey- 

 Wyssling and Frey, 195 i) interwoven crossed microfibrils are visible 

 (Fig. 86d, p. 128). Therefore, in one and the same microscopic 

 lamella, fibrillar elements may be laid down in different directions and, 

 if they cross at 90°, their optical anisotropy is mutually cancelled when 

 polarized light is transmitted perpendicularly. 



Rodlet and intrinsic birefringence of the chi tin frame. Chi tin was the first 

 biological object in which the interaction of textural and intrinsic 

 birefringence — discovered by Ambronn in artificial gels — could be 

 demonstrated (Mohring, 1922). When decalcified specimens of 

 chitin (lobster shell or lobster tendons) are immersed in solutions of 

 potassium mercuric iodide of increasing refractive power, the bire- 

 fringence decreases, falls to nil, changes its sign, reaches a minimum 

 in the negative region, becomes nil a second time and then returns 

 to positive (Fig. 151). The inference from this is that chitin is marked 

 by a pronounced positive form anisotropy, i.e. rodlet birefringence, 

 and a sHghtly negative intrinsic birefringence. 



In his imbibition experiments Castle (1936) finds reversal of the 

 birefringence with mercuric iodide of potassium and iodobenzene in 

 xylene, but not with other organic liquids (methylene iodide in 



