50 KERATIN AND KER ATINIZATION 



deposits in the cytoplasm might be found in the cytoplasmic fibrils 

 beneath the plasma membranes in many generalized cells. Possibly too, 

 they share a common molecular ancestor with the contractile muscle 

 proteins and other intracellular fibrillar systems of the same basic molecular 

 type (see p. 22), but these views must remain conjectural in the present 

 state of our knowledge. Such cytoplasmic fibrous proteins are not strictly 



WATER+ SALTS 



SEMIPERMEABLE 

 MEMBRANE 



IMPERMEABLE 

 CUTICLE 



EXCRETORY 

 ORGAN 



HYPOTONIC 

 URINE 



Fig. 24. An osmoregulatory mechanism depending on the development 



of a water impervious cuticle limiting input and output of water to 



small areas. From Baldwin (1937) with permission. 



keratin in the sense of a definition restricting this term to stabilized fibrils 

 although their role may have been protective. An accepted opinion 

 relates the early specializations of the outer layers to the problem of water 

 control rather than mechanical protection. It is supposed that the need 

 for such control arose in the first place with the colonization of fresh- 

 water habitats (Baldwin, 1937). Animals leaving the sea, where the osmotic 

 pressures inside and outside their cells were nearly equal, and entering 

 fresh water, where the salt concentration was less, would be faced with 

 stresses owing to the entry of water into their cells. They would therefore 

 be forced to evolve specialized organs to control the entry and egress of 

 water. At the same time by enclosing themselves in a waterproof coat and 

 thus limiting their water exchanges to a small area (Fig. 24), the work 

 required of the excretory organ would be lessened. At first, if we take 

 the condition in present-day fish as a clue, the epidermal layer was 

 supplemented by the secretion of a slimy mucilagenous layer ; this device 

 has, however, tended to lessen in importance as the degree of keratinization 

 of the cells themselves increased. 



