26 KERATIN AND KERATINIZATION 



to construct a geneological table for the major macromolecular species. 

 As soon as this is attempted, however, it becomes plain that, as stated 

 above, in its grand features biochemical evolution has been small relative 

 to structural. The major phyla are distinguished certainly by their 

 characteristic spectra of macromolecules (Fig. 13) but very clearly this 



Fig. 14. The surface specializations of cells and their role in tissue 

 building and in the formation of an integument, (a) Generalized cell; 

 (b) isolated cell surrounded by a layer of mucin which may form a 

 pellicle ; (c) isolated cell with surface covered with motile cilia ; (d, f ) one 

 theory of the formation of a multicellular organism. Failure of cells 

 to separate after division owing to the adhesions of the superficial coats; 

 (e, f) alternative view of the origin of multicellularity, the cellularization 

 of a large cell (Hadzi, 1954). This procedure is followed in the subdivision 

 of the egg. (g) and (h) The two types of protective integument : (g) the 

 extracellular cuticle or pellicle secreted by the surface epithelium. This 

 must be shed to permit growth ; (h) an integument formed by the surface 

 layer of cells themselves which have synthesized a fibrous intercellular 

 material; this is the case in the keratinized tissues. 



sharing-out of molecular types took place at a time anteceding anything 

 of which we have fossil evidence. The structural evolution of which we 

 have evidence has represented a shuffling of possibilities presented by a 

 more-or-less constant molecular endowment. Nothing may give a more 

 vivid impression of the unity of life than the recognition throughout its 

 manifestations of similar molecular species and of similar biochemical 

 devices based upon them (Florkin, 1960). 



Since the distribution of macromolecular types occurred so early, a 

 consideration of this distribution and its relation to function should offer 

 a promising pathway to investigate the obscure field of primordial life and 



