CH. V] OF THE COLLOID STATE 347 



in connection with form, throwing light on what seem to be common 

 characteristics and pecuHarities of the forms of Hving things. 



Much has been done, and more said, about the nature of protoplasm since 

 this book was written. Calling cytoplasm the cell-protoplasm after deduction 

 of chloroplasts and other gross inclusions, we find it to contain fats, proteins, 

 lecithin and some other substances combined with much water (up to 

 97 per cent.) to form a sort of watery gel. The microscopic structures 

 attributed to it, alveolar, granular or fibrillar, are inconstant or invalid; 

 but it does appear to possess an invisible or submicroscopic structure, 

 distinguishing it from an ordinary colloid gel, and forming a quasi-solid 

 framework or reticulum. This framework is based on proteid macromolecules, 

 in the form of polypeptide chains, of great length and carrying in side-chains 

 other organic constituents of the cytoplasm*. The polymerised units 

 represent the micellae f which the genius of Nageli predicted or postulated 

 more than sixty years ago; and we may speak of a "micellar framework" 

 as representing in our cytoplasm the dispersed phase of an ordinary colloid. 

 In short, as the cytoplasm is neither true fluid not true solid, neither is it true 

 colloid in the ordinary sense. Its micellar structure gives it a certain rigidity 

 or tendency to retain its shape, a certain plasticity and tensile strength, a 

 certain ductility or capacity to be drawn out in threads; but yet leaves it 

 with a permeability (or semi-permeability), a capacity to swell by imbibition, 

 above all an ability to stream and flow, which justify our calling it "fluid 

 or semi-fluid," and account for its exhibition of surface-tension and other 

 capillary phenomena. 



The older naturalists, in discussing the differences between organic and 

 inorganic bodies, laid stress upon the circumstance that the latter grow by 

 "agglutination," and the former by what they termed "intussusception." 

 The contrast is true; but it applies rather to solid or crystalline bodies as 

 compared with colloids of all kinds, whether living or dead. But it so happens 

 that the great majority of colloids are of organic origin; and out of them our 

 bodies, and our food, and the very clothes we wear, are almost wholly made. 



A crystal "grows" by deposition of new molecules, one by one 

 and layer by layer, each one superimposed on the solid substratum 



* See {int. al.) A. Frey-Wyssling, Subrnikroskopische Morphologie des Protoplasmas, 

 Berlin, 1938; cf. Nature, June 10, 1939, p. 965; also A. R. Moore, in Scientia, 

 LXii, July 1, 1937. On the nature of viscid fluid threads, cf. Larmor, Nature, 

 July 11, 1936, p. 74. 



f Micella, or micula, diminutive of mica, a crumb, grain or morsel — ynica panis, 

 salis, turis, etc. Nageli used the word to mean an aggregation of molecules, as 

 the molecule is an aggregation of atoms; the one, however, is a physical and the 

 other a chemical concept. Roughly speaking, we may think of micellae as varying 

 from about 1 to 200 /x/x; they play a corresponding part in the "disperse phase" 

 of a colloid to that played by the molecules in an ordinary solution. The macro- 

 molecules of modern chemistry are sometimes distinguished from these as still 

 larger aggregates. See Carl Nageli, Das Mikroskop (2nd ed.), 1877; Theorie der 

 Gahrung, 1879. 



