KERATIN AND MOLECULAR BIOLOGY 



23 



layers of the embryo come to produce and to retain intracellularly the 

 contractile muscle protein (actomyosin) and thus become muscle cells; 

 others of a similar origin become fibrocytes and by secreting the fibrous 

 protein collagen help to build the connective tissues of the organism and 

 the lower layer of the integument. The superficial cells, on to which 

 devolves the special task of enclosing the whole system, commence to 

 differentiate early and lay down intracellular keratin. 



The second distribution, that of the macromolecules among the different 

 phyla, is a fascinating problem with far-reaching implications for their 



FlG. 12. Cross-section of an hypothetical vertebrate body to show dis- 

 tribution of fibrous proteins. The three major divisions are shown 

 schematically. E, the external cellular epidermis (ectoderm) containing 

 keratin fibrils K; the internal epithelia I (endoderm) are separated 

 by the middle layers (mesoderm) containing the musculature M and the 

 connective tissue (including dermis) containing collagen fibrils C. 

 Basement membranes BM (see p. 86) separate the epithelia from the 

 mesodermal layers. 



evolutionary development. It would seem that the power to make protein 

 and/or polysaccharide materials which can be hardened by various 

 chemical devices is a primitive and persistent cell property (Fig. 16). 

 Organisms can call upon this property if and when necessary to form 

 integuments, egg cases and other hard parts. Thus no necessary cor- 

 respondence between phylogenetic relationships and the distribution 

 of hardened parts can be insisted on. Nevertheless in the event 

 some persistent trends exist. 



