48 



FUNDAMENTALS OF CYTOLOGY 



chains. When such long molecules or molecular chains have a random 

 arrangement, the system is isotropic. When they lie closely parallel 

 and are free to move upon each other, they are in the paracrystalline, or 

 "liquid-crystal," state, and if they lie in a medium of different refrac- 

 tive index, the system exhibits double refraction. In the gel state, 

 characterized by considerable firmness and elasticity, the chains evidently 

 tend to associate in a sort of network or to unite closely here and there into 

 bundles, or micelles. Such elastic gels may quickly liquefy when jarred 

 or stirred owing to a property known as thixotropy. 



In such phenomena the cytologist finds welcome clues to the sub- 

 microscopic structure of protoplasm. For example, double refraction 

 can be detected in muscle cells, plastids, chromosomes, and mitotic 

 spindles; furthermore, ice crystals formed \\'ithin these structures tend to 



NH. 



COOH NH 



RCH NHH COOH 



Union of two amino ocids with loss of HiO 



RCH 



Polypeptide chain of indefinite lengtti 



Fig. 36. — The constitution of the polypeptide chain. The symbol R stands for side chains 



of several kinds: NH2, COOH, SH, OH, etc. {Adapted from Frey-WyssHng.) 



be oriented accordingly. Gelled regions in protoplasm frequently pass 

 into the sol state when stirred with the micromanipulator needle. Proto- 

 plasm is elastic. A certain myxomycete Plasmodium can pass through 

 pores Iju in diameter slowly and by itself, but it is destroyed if pressed 

 through gauze with openings 200^1 in diameter. All such observations, 

 together with the visible fibrous differentiations more directl}^ observable, 

 point to the conclusion that protoplasm has a fibrous constituent in its 

 fundamental structure. 



It is now thought probable that the submicroscopic fibrous constituent 

 of protoplasmic structure consists primarily of proteins. Researches on 

 proteins show that their molecules consist of amino acids arranged in the 

 form of long polypeptide chains (Fig. 36). Such a chain may be extended 

 as a "fibrous molecule" which could, with a molecular weight of 35,000, 

 reach a length of 0.1 m- The results of studies with X rays and polarizefl 

 light show that molecules in this extended form, singly or in bundl(\s 

 (micelles), are present in tissues with mechanical functions (muscles, 



