42 PROTOPLASM AND THE CELL 



hormone is secreted which reaches the epithelial cells of the 

 uterus through the blood and causes these cells to react to the 

 embryo. Again the pituitary body in the brain plays an im- 

 portant part in regulating growth of the organism, diseases of 

 this gland giving rise to acromegaly, one of the symptoms of 

 which is the excessive development of parts of the organism far 

 removed from the brain. 



Endoenzymes. The third type of enzyme action, that due to 

 endoenzymes, involves even more subtle chemical activities 

 among the molecules of protoplasm. These elusive bodies are 

 the chief elements in the growth and breakdown of protoplasm, 

 i.e., in constructive and destructive metabolism. They are 

 best known in connection with destructive metabolism, the 

 modern conception being that endoenzymes are the causes 

 of a series of progressive chemical decompositions. Each 

 chemical process is presided over by a specific endoenzyme 

 which acts only on certain chemical substances and gives rise 

 "to other chemical substances to be acted on in turn by other 

 enzymes. "The processes which years ago were considered 

 as due to the peculiar vital properties of the tissue cells, and 

 which were supposed to be entirely dependent upon their 

 morphological and functional integrity, are now seen to be due 

 primarily to a great variety of enzymes, manufactured indeed 

 by the living cells, but capable of manifesting their activity 

 even when free from the influence of the living protoplasm. 

 The varied processes of tissue katabolism are the result of or- 

 derly and progressive chemical changes, in which cleavage, 

 hydrolysis, reduction, oxidation, deamidization, etc., alternate 

 with each other under the influence of specific enzymes, where 

 chemical constitution and the structural make-up of the various 

 molecules are determining factors in the changes produced." 

 (Chittenden, The Nutrition of Man, pp. 75, 76.) 



