182 E. UHLENHUTH 



of the planj. He was able to indicate the existence of specific substances 

 necessary to the production of leaves and roots, and showed that quantita- 

 tive relations exist between the organ and the substance producing it; 

 more and larger organs are produced as a greater quantity of the sub- 

 stance is supplied at the place of growth. 



Little is known as to the mode of action of the internal secretions, al- 

 though we do know, through the discoveries of Kendall, that the thyroid 

 hormone acts as a catalyser and augments the metabolic process in an 

 effective and specific manner. If this be the only peculiarity of the thyroid 

 hormone, by means of which it produces new structures, it may mean 

 that mere quantitative differences are perceived by our sense organs as 

 qualitatively different morphological structures. This view fits in well 

 with observations of Child on diverse groups of organisms. According 

 to this author, the qualitative differentiations along the various axes of 

 an organism are simply the expression of quantitative differences of metab- 

 olism. For instance, the formation of the head, according to Child, takes 

 place wherever in an organized living system a metabolic maximum is 

 formed. 



Among the endocrin glands only the thyroid, the pituitary body, and 

 the sex glands are definitely known to control growth or development, or 

 both. These three glands and the thymus will be discussed in this article. 

 Although the thymus gland seemingly does not actually play any role in 

 growth and development, it has been thought well to include it here be- 

 cause of a widespread belief that it has such a function. 



The Relation of the Thyroid to Growth and Development 



In the quadruped vertebrates the presence of the thyroid gland is 

 absolutely necessary for normal growth and development of the organism, 

 though the absence of this gland, at least in the lower quadrupedes is not 

 incompatible with the maintenance of life. It is probable that not only 

 the ontogenetic development of the individual is controlled by the thyroid 

 hormone, but that even the phylogenetic development of a large group of 

 organisms containing the most highly organized animals and man was 

 initiated through the acquisition, by the amphibia, of the thyroid mechan- 

 ism in the condition as we find it to-day in these animals. 



Before entering upon the subject it is perhaps well to point out that 

 most of the morphogenetic effects brought about by the action of the 

 thyroid can best be understood if we assume that they are the particular 

 response of the inherited physicochemical structure of the organisms to 

 an agent having the peculiarity of increasing the rate of chemical reac- 

 tions more than any other known substance. That the thyroid hormone 



