52 KERATIN AND KERATINIZATION 



morphosis. Some authors have reported free SH groups in the amphibian 

 stratum corneum. 



Obviously the discovery of some method of hardening, reducing the 

 swelling and waterproofing the integument is a prerequisite for life in a 

 dry environment and, in this sense, the process of keratinization is rightly 

 described as one of the key biochemical discoveries on whose exploitation 

 the success of the vertebrates is based. In other land-dwelling forms 

 different macromolecules have been adapted to meet these same needs, 

 and, as mentioned above, the nature of the structural macromolecules found 

 in the integument places certain limitations on the evolution of a phylum. 

 A more fully-developed knowledge of molecular phylogeny may be able 

 to relate the mutations, which gave rise to the various molecular types to 

 the subsequent evolutionary development. 



