KERATIN AND MOLECULAR BIOLOGY 51 



If we accept this hypothesis of the origin of the strengthened and water- 

 proofed integument, then we may regard its evolution in fresh water as a 

 fortunate pre-adaptation without which a subsequent colonization of the 

 more unfavourable environment, the dry land, could not have been 

 attempted. However this may be, it was on the land where the full 

 possibilities of keratinization were revealed and the astonishing variety of 

 modified skins, claws and scales were evolved from epidermal thickenings 

 culminating in the appearance of feathers, the distinguishing mark of 

 birds, and of hairs, equally characteristic of mammals. Since these 

 hardened parts may leave fossil imprints and since a sufficient variety of 

 animal types has survived until today, this later evolution is reasonably 

 well documented. 



The greatest degree of keratinization was reached in reptiles; in birds 

 and mammals, with the elaboration of feathers and hairs, the thickness and 

 degree of keratinization of the epidermis itself lessens. 



In most organisms the waterproofing properties of the toughened 

 framework formed by the structural macromolecules (proteins and/or 

 polysaccharides) are supplemented by the addition of lipid materials. 



An increase in the degree of stabilization of the epidermal protein 

 itself appears to have occurred in higher vertebrates, if we may judge 

 from the skins of surviving types. An experimental measure of the degree 

 of stabilization may be obtained by observing the temperature at which 

 an oriented fibrous system contracts on heating due to the shortening of 

 its molecular chains by thermal agitation (see p. 255). Using this method 

 Rudall (1955) has shown more precisely that in the case of the newt, 

 Triturus, the stability of the skin was intermediate between that of a 

 film of myosin, an unstabilized muscle protein of the a-type, and human 

 leg stratum corneum. The temperature at which the X-ray pattern became 

 disoriented or was converted into a /J-type pattern was also deter- 

 mined by Rudall and again demonstrated an intermediate degree of 

 stabilization. 



The amphibians, in this respect as in others, are " living fossils " and 

 preserve, in the changes which take place in their epidermal cells at 

 metamorphosis, a suggestion of the course of evolution of the keratinized 

 skin of the fully land-dwelling animals. The larval skin contains a variety 

 of cells, some ciliated, some secreting mucins and others containing 

 masses of fine fibrils, from which it is possible to infer that the factors 

 determining complete keratinization are not yet present. Histochemical 

 tests (Barrnett, 1953) based on the demonstration of cystine cross-linkages 

 as stabilizing elements (Hergersberg, 1957) show almost a complete 

 absence of true keratinization; after metamorphosis, when the animals 

 become capable of living out of water, the epidermis becomes keratinized. 

 The change is also provoked by thyroxin which causes premature meta- 



