Biochemical and Biophysical Phenomena 769 



have demonstrated the presence of organic hormones In such inverte- 

 brate animals as insects, worms, crustaceans, cephalopods, etc. It is be- 

 lieved that invertebrates produce certain hormones, many of which 

 resemble those of the vertebrate animals, whose functions approximate 

 those of higher animals. 



Vitamins 



The definite chemical composition of certain vitamins is now fairly 

 well established. Certain vitamins are rather unstable, especially when 

 subjected to heat, oxygen, or light. Vitamins may be present in all 

 living protoplasm in minute amounts, but in larger quantities they are 



Fig. 369. — Effect of vitamin A on the growth of young white rats. These two 

 rats, from the same htter, received the same food except that the one on the left 

 had its allowance of vitamin A reduced. (Courtesy of Parke, Davis and Co.) 



r 



Fig. 370. — Xerophthalmia, an eye disease caused by a dietary deficiency of 

 vitamin A. The eye becomes dry and a layer of horny tissue forms upon the 

 cornea. (From Harris: Vitamins in Theory and Practice, 1935. Courtesy of the 

 Cambridge University Press and The Macmillan Company.) 



