KERATIN AND MOLECULAR BIOLOGY 49 



ance and function. Examples are numerous secretory granules (enzymes 

 and hormones), pigment granules and fibrils of various sorts including 

 keratin. 



It may seem significant that the number of basic structural elements 

 at the macromolecular level is so small. To some the situation would 

 seem to be a strong argument in favour of supposing a common descent 

 from an archetypal cell in which such devices as cilia and mitochondria 

 were already present. To others, and this may be the more austerely 

 biomolecular view, these resemblances in the primary organelles 

 indicate no more than that, given the limited molecular materials available 

 (proteins, phospholipids, polysaccharides, etc.), the number of structural 

 solutions to such problems as: enclosing whole cells, segregating intra- 

 cellular catalysts and a genetic apparatus, the provision of surface organs 

 of motility and a protective integument, is limited. The actual devices 

 found are effective and perhaps the only solutions to the problems. 

 While we can scarcely hope to discover how these structures came into 

 being, an experimental demonstration of their formation in vitro is 

 certainly conceivable as recent experiments on phospholipids show 

 (Stoeckenius, 1959; Mercer, 1960). In the same way the limited number 

 of fibre-types reflects the few possible ways of folding and packing poly- 

 peptide chains. These are limitations at the chemical level. Nevertheless 

 the tendency to produce certain molecular species and to use them in a 

 given situation is inherited and has phylogenetic significance (p. 22). 



The phylogeny of keratinization 



In the form found among existing land animals the epidermis is the 

 culmination of a long evolution on dry land, the steps of which can be 

 reconstructed from its histology among these animals (Romer, 1955 and 

 Young, 1950). Obviously this evolution commenced before the dry land 

 was invaded since many fishes deposit intracellular fibrils in their epidermal 

 cells and there are instances, as in the horny teeth of lampreys (Barrnett, 

 1953), of localized deposits of hardened protein among the lower verte- 

 brates. Also, from their almost universal distribution, there is every reason 

 to suppose that the group of cell responses associated with the surface 

 membrane, which were described in the previous section, can be traced 

 back to the earliest free-living cells. It is not without interest to try to 

 trace back the origin of a specialized epidermis to more remote beginnings, 

 to consider it as part of the more general problem of the evolution of 

 protective layers in organisms. What lends a particular interest to such 

 speculations is the possibility already mentioned that the events at the 

 surface of cells may have played decisive roles in initiating the various lines 

 of evolution. 



We have pointed out that the beginnings of such supporting fibrillar 



