PRINCIPLES OF STRl^CTURK 



55 



from the alkali salts of oleic acid when these are wetted. Very beautiful 

 myelin forms were obtained by Gicklhorn (1932a) in the cell sap of 

 the well-known AlUuni epidermal cells by adding ammonia or sodium 

 hydroxide (Fig. 46). The variety of shapes in these peculiar structures 

 is beautifully demonstrated in Nageotte's microphotograph atlas 

 (1936, No. 434). 



The myelin forms are usually designated as liquid crystals. It 



Fig. 46. Myelin forms in the epidermal cells oi Allium (from Gicklhorn, 1932a). 



should be pointed out, however, that there is a fundamental difference 

 between these structures and the crystalline liquid state mentioned 

 above. For, in the latter we have to deal with a special aggregate 

 state of a uniform substance, i.e., a system consisting of one com- 

 ponent only, whereas at least two components take part in the formation 

 of myelin forms. In the examples mentioned, one of these components 

 is water. It is further essential that the molecules, which here again 

 must have a chain-like structure, be not homopolar as in Fig. 45, but 

 heteropolar, i.e., they must contain a hydrophilic and a lipophilic 

 pole. The hydrophilic group in oleic acid is the carboxyl group, that 

 in lecithin is the choline. If the conditions mentioned are realized, 

 myelin forms may occur, provided the molecules are sufficiently mobile. 

 The apparent growth is due to water absorption; it is, therefore, a 

 matter oi swelling: the hydrophilic groups are surrounded by water, 

 while the hydrophobic groups are drawn away from the surface. The 

 resulting orientation in the case of lecithin is represented in Fig. 47a; 

 the lecithin underlying this scheme is a /5-lecithin (see Fig. 93, p. 138) 

 in which the phosphoric acid is attached to the OH-group in the middle 

 of the glycerol molecule. Obviously, the water penetrating into the 



