22 KERATIN AND KERATINIZATION 



the new pattern may be that of disoriented jS-crystallites, which may be 

 oriented by stretching and then give an oriented jS-pattern. 



Ecdysis. One consequence following from the fact that the chemical 

 changes involved in stabilization are irreversible, is that provision must be 

 made for the removal of the hardened layer to permit further growth. This 

 may take the form of moulting or casting when an entire covering of 

 feathers, scales, shell, etc., may be lost or, in the case of the keratinized 

 cellular epidermis, superficial cells may be constantly shed and a covering 

 of constant thickness maintained— a procedure which has the advantage of 

 not exposing its owner to a period of vulnerability such as follows the 

 moulting of a rigid covering. The factors governing ecdysis are obscure ; 

 they seem to be hormonal and are often geared to seasonal changes and, 

 in turn, to other cyclic hormonal-controlled activities, such as sexual 

 display, in which the external coverings may play a conspicuous part 

 (pp. 133 et seq.) (Turner, 1960). 



Distribution of the fundamental fibre-types 



There are two distributions to be considered : the first is the distribution 

 of the different macromolecular types among the parts of any particular 

 kind of organism, which raises questions of ontogeny; the other is the 

 sharing-out of molecular types between the entire range of organisms and 

 here there are problems of phylogeny. 



The first of these distributions is linked with the problem of the 

 differentiation of organs during embryogenesis which is usually recognized 

 and defined by the appearance of the typical histology of the various 

 tissues as seen in the light microscope. These changes at a relatively 

 large-scale level are in fact partly the consequence of the appearance and 

 accumulation in the developing tissues of the characteristic macromolecules 

 under discussion. Differentiation may thus be described in terms of the 

 macromolecules on whose presence the future function of the tissue 

 depends. 



In very generalized terms, the fertilized egg is biochemically and 

 structurally omnipotent; it is potentially capable of synthesizing all the 

 products later appearing in its descendant cells. As development proceeds, 

 these potentialities are shared out among the organ systems, each of which 

 finally makes a limited range of substances required for their special 

 function. Thus in the adult, many of the tissues may be characterized 

 by the fact that they contain a limited range of structural macromolecules 

 of which a few (perhaps only one) associated with the tissue's function, 

 greatly predominate in amount. 



This is well illustrated by the sharing out of the fibre-forming potential- 

 ities in the vertebrates (Fig. 12). Certain groups of cells in the middle 



