KERATIN AND MOLECULAR BIOLOGY 27 



its dispersion into phyla. What is of particular interest here, since it is 

 our object to discuss the integument, is the possibility that the molecular 

 basis, both for the appearance of multicellularity and for the separation of a 

 limited number of well-defined types of organism, is to be sought for in 

 the nature of the substances present on the surface of cells (Fig. 14). 

 These substances determine in the first place the intercellular adhesion, 

 the essential basis of the existence of cells in colonies (see Fig. 14(d), 

 (e) and (f)) and, in the second place, the types of material from which 

 superficial cells construct their protective layers, is correlated with the 

 divergent lines of evolution in such a way as to suggest the choice represents 

 a major cause of this dispersion. 



The chemical processes and materials responsible for toughening tissues 

 are also distributed in a phylogenetically significant way. For example, 

 among plants we normally find the fibrous, polysaccharide cellulose 

 embedded in various encrusting substances such as pectins and lignins; 

 among the arthropods, we find the polysaccharide, chitin, embedded in a 

 tanned /2-type protein (Richards, 1951). Tanning is more or less univer- 

 sally distributed among animals and thus must be judged the more 

 primitive device for cross-linking protein chains. According to Mason 

 (1955) its wide distribution is related to that of the ubiquitous enzyme 

 systems, phenolases, which catalyse the formation of o-quinones from 

 phenols. Tanning seems to be the rule in invertebrates; in vertebrates 

 the process is found mainly in one kind of cell, the pigment producing 

 melanocyte, where pigment granules are darkened and hardened by the 

 formation of tanned melanoproteins (see p. 276). A fundamental difference 

 in body plan with related mechanical consequences thus arises between 

 the invertebrates, with their rigid, tanned exoskeleton on the one hand, 

 and the vertebrates, with a rigid endoskeleton of collagen and calcium 

 salts and a more flexible keratinized epidermis on the other. Some further 

 consideration of the phylogeny of keratinization itself among the vertebrates 

 will be given on p. 49. 



The relationship, between certain fundamental structural " inventions " 

 and the molecular bases on which they rest, is set out in tabular form in 

 Fig. 16. 



Some difficulties in denning a keratin 



The definition of a keratin assumed above is that it is a hardened and 

 insolubilized protein found within the epidermal cells of vertebrates. 

 This definition covers almost all the proteins which will be discussed in 

 this work, but it certainly does not cover all those which might, on one 

 ground or another, be considered as having a claim to the name. There- 

 fore it is desirable to consider alternative definitions based on some 

 characteristic molecular structure or chemical feature. The mammalian 



