DIFFERENTIATION AND PROTEIN SYNTHESIS 123 



keratin may even be secreted. The cells lining the gizzard produce a horny 

 lining of keratin (p. 108) and the oviduct covers the egg with a mixed 

 mucin and keratin layer which forms the tough membranes immediately 

 underlying the shell (Plate 18B). 



The supermolecular organization of fibrous tissues 

 (tertiary structure) 



Macromolecular Fibrous Texture 



We have up to this point been considering those aspects of the synthesis 

 of fibrous proteins which they share in common with the soluble proteins. 

 However, the special characteristic of fibres is the supermolecular aggre- 

 gates which they form whose dimensions may extend from the molecular 

 to the histological level and beyond. These aggregates often display a 

 remarkable and intricate structure at several levels and it is a further 

 problem to give an account of the genesis of this larger-scale organization. 



Fibres usually perform a mechanical function; i.e. they transmit 

 tension, strengthen membranes or provide against impact, and their 

 organization within tissues is normally related to these functions. In fact 

 some of the most striking examples of biological adaptation are provided by 

 the fibrous tissues. It is more useful in this connexion to speak of a 

 fibrous texture rather than of fibres and thus to focus attention on the 

 microscopic rather than the macroscopic elements of structure. 



In a purely geometrical sense we may list the possible varieties of 

 fibrous texture as in Fig. 53. 



Organisms exploit all these possible arrangements and it is not usually 

 difficult to relate the textures to the mechanical function. The difficulty 

 is to account for the origin of the structure in molecular terms, particularly 

 in instances where it appears apparently in advance of function. 



When studying a complex organism in its structural or functional 

 aspects, it is convenient to distinguish a hierarchy of levels of organiza- 

 tion. At each level new possibilities of organization are introduced 

 and the study of the structure as a whole is facilitated if the events at each 

 of the several levels can be considered separately. Fibrous tissues lend 

 themselves to this form of analysis. They exhibit, as Astbury has put it, 

 " patterns within patterns". In this sense we shall distinguish here three 

 levels: the molecular, the macromolecular and the microscopic level, which 

 happen also to be those which correspond to structures of orders of 

 magnitude most easily studied by X-ray crystallography, electron micro- 

 scopy and light microscopy respectively (Fig. 1) ; but, although convenient, 

 there is no significance in this correspondence, which is in any case inexact 

 and not lasting, since the domains of X-ray diffraction analysis and electron 

 microscopy increasingly overlap. The structures existing at each level are 



