HIGHER INVERTEBRATES AND AMPHIBIA 233 



senting various kinds of inductors exist and are transferred from one part 

 of the body to another; these, acting on a specific substratum of a very plastic 

 nature, are presumably responsible for the production and growth of the 

 organ systems which are characteristic of the different regions of the body. 

 In arthropods such transformations are limited to certain appendages, and 

 also in urodele amphibia integrative processes are possible only within a very 

 limited range; but in anuran amphibia they are lacking altogether. There 

 develop in insects and in the higher organisms, in accordance with their more 

 finely differentiated organs and tissues, hormones and neuro-hormones which 

 affect certain organs and cells in a very specific manner. Such hormones may 

 also affect the life and growth of transplanted organs with which they have 

 specific relations. Perhaps a corresponding increasing differentiation exists 

 also in the case of various other substances, such as vitamins and enzymes, 

 which regulate maintenance, growth and metabolism of organs and tissues; 

 but we have as yet no definite knowledge as to the phylogenetic development 

 of the latter types of substances. 



