Hormone Factors in Growth and 

 Development 



E. UHLENHUTH 



NEW YORK 



Introduction 



Among the problems that present themselves in the study of living 

 organisms, one of the most fundamental, fascinating, and yet obscure is 

 that regarding the mechanism involved in the production of organic form. 

 Why does living matter assume the forms and structures that it does? 

 Important progress has been made in recent years in elucidating the re- 

 lationship existing between life and the visible form to which it attaches 

 itself. Most helpful in this respect was the knowledge gained by the dis- 

 covery of the internal secretions, through which it became known that 

 definite organic structures are related to definite specific substances. The 

 greatest progress along this line came from the discovery of the thyroid 

 hormone, the relation of which to growth and development is well estab- 

 lished. According to the knowledge which we possess at present, it would 

 seem that a whole series of new morphological structures arose, phylo- 

 genetically speaking, in the amphibians as a result of the thyroid hormone, 

 and it is probably upon this basis that the entire development of the higher 

 vertebrates has proceeded. This instance will serve to illustrate the im- 

 portance of the internal secretions in the development of organisms. Simi- 

 larly well known is the establishment of the secondary sex characters 

 through the action of the sex hormones. It is certain that internal secre- 

 tions played a large role in the formation of animal organisms ; rudiments 

 of the endocrin glands are found already in the crustaceans (Patten), 

 and there are reasons to assume the action of internal secretions in 

 organisms as low as the insects (Bordage). 



It should be kept in mind, however, that the internal secretions con- 

 stitute only one group of specific substances, the morphogenic action of 

 which has been recognized as one of the main principles in the develop- 

 ment of organisms. The important role of specific substances in the 

 morphogenesis of the organisms was emphasized by the botanist Julius 

 Sachs, and recently by Jacques Loeb. In the experiments of the latter 

 made on Bryophyllum he demonstrated this role as regards various organs 



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