INSECT CUTICLE — RICHARDS AND PIPA 



253 



such sections to hot lo-percent NaOH sohition for 20 minutes abol- 

 ished the birefringence, i.e., cross sections became completely isotropic. 

 This is strong evidence — it could, indeed, be called proof — that the 

 arthropodin particles in normal cuticle are oriented at a right angle to 

 the chitin chains. 



Incidentally, as was to be expected from the magnitude values given 

 above, the birefringence of a cross section of tendon is much lower 

 than the birefringence of a longitudinal section. This is shown by 



Fig. 2. — Diagrams of chitin and artliropodin chains. Chitin molecules drawn 

 as straight lines, arthropodin molecules drawn as zigzag lines. A, The parallel 

 chain suggestion of Fraenkel and Rudall (1947). B, The cross-grid possibility 

 listed by Rudall (1950) and verified herein. C, The distorted cross grid pro- 

 posed herein; viewed along the c axis. D, The same, but viewed along the 

 b axis. 



relative brightness of the low magnification pictures of a cross section 

 and a longitudinal section of normal tendon presented in plate i, 

 figure 8. If one could be sure that the sections were of the same 

 thickness, they would permit ready determination of accurate values 

 for the birefringence of chitin, arthropodin, and cuticle. But, since 

 the thicknesses of our sections were probably only approximately the 

 same, we can only say that the value for longitudinal sections is a 

 number of times greater than that for cross sections and approxi- 

 mately what one would expect from the numerical values given in 

 preceding paragraphs. 



