KERATINIZATION 7 



formation of this pigment. These elements of course lie near the 

 blood vessels and have ample mitochondria and Golgi apparatus. 

 In them the melanin arises almost certainlv from a colourless 

 precursor, leuco-melanin. The coloured granules lie between the 

 nucleus and the surface of the body ; perhaps Ludford's suggestion 

 that this is a mode of protecting the nucleus from the injurious 

 effects of strong ultra-violet light is an explanation of the arrange- 

 ment. So far we are on fairly safe ground ; the controversy 

 starts in connection with a second mode of formation of pigment 

 in certain elements of irregular shape : the melanoblasts. These 

 lie only in the stratum germinativum and their processes extend 

 between the neighbouring epithelial cells. The word elements has 

 been used here instead of the more usual term cells, because it is 

 quite possible that we are really dealing simply with inter- 

 cellular spaces rendered visible by being filled with granules of 

 pigment. 



The process of keratinization has not been adequately explained, 

 and even a statement of the sequence of events leading to the 

 production of keratin is inevitably incomplete. The change 

 occurs as the cells migrate further and further away from their 

 blood supply and presumably become more dependent upon slow 

 methods of nutrition, viz., by diffusion through the inter-cellular 

 spaces. Keratinization might therefore be the result of mal- 

 nutrition or of the difficulty of complete removal of waste sub- 

 stances from the cell body. The latter view is supported by 

 Drew's observation that keratinization will occur in tissue cultures 

 of skin if subcultures are not made at sufficiently short intervals. 

 Further, he showed that the process of formation of the horny 

 substance proceeded most actively and efficiently in the presence 

 of connective tissue. Obviously the cells of the skin, when 

 becoming keratinised, are far removed from any influence likely 

 to be exerted by mesoblastic tissue. 



STOMACH 



Perhaps one of the most important regions to which much 

 investigation has been devoted is the lining of the stomach. 

 Obviously the human stomach is but rarely in a suitable state for 



