202 E. UHLENHUTH 



of the thyroid and that the thynms produces specific growth-promoting 

 substance^ on the one hand and specific development-inhibiting substances 

 on the other, the latter inhibiting the excessively rapid differentiation 

 observed in thyroid-fed larva?. Abderhalden has recently propounded the 

 same view, though the evidence he could furnish in favor of it seems 

 very unsatisfactory. 



Lately very careful experiments have been carried out with a view 

 to testing the validity of the conclusions derived from the older investiga- 

 tions. These experiments almost unanimously have brought to light facts 

 demonstrating that the thymus gland does not control either growth or 

 development, Nordman, who repeated the extirpation experiments on 

 dogs, was the first to deny that extirpation of the thymus retards growth 

 in a specific way or produces disturbances in the ossification of the 

 bones. He considered that such effects were due, in the experiments car- 

 ried out by previous authors, not to the lack of the thymus function in the 

 thymectomized animals, but to crowded enclosures and insufficient food. 

 Similarly, Pappenheimer, Holnan and Marshall and Eenton and Robert- 

 son could not observe any disturbance in growth and development after 

 thymus removal. Recently Park and McClure published their very care- 

 ful and critical experiments conducted on dogs and on a large scale ; they 

 show that normal growth takes place after thymus removal and prove that 

 the development of the skeleton as well as of the testes of the thymecto- 

 mized animals is normal. In amphibians (tadpoles), too, the removal of 

 the thymus gland in early embryonic stages does not interfere with normal 

 growth and development, at least during the larval period. (Adler (5), 

 Allen (e). 



Similar results were obtained in the feeding experiments. In 1916 

 Hoskins fed thymus to rats, but did not notice any effect on growth and 

 development. Lately Uhlenhuth (a) (b) (f) (g) has carried out thymus 

 feeding experiments in large numbers of the larvae of salamanders. These 

 experiments gave the following results : If small amounts of thymus are 

 added to the diet the larvas grow and develop at the same rate as the con- 

 trols. If large amounts of thymus are added to the diet, metamorphosis 

 is retarded pari passu with the amount of thymus administered. Not only 

 development, however, is retarded, but also the rate of growth is slowed 

 down. Animals that are fed thymus exclusively grow and develop only 

 for a short time after hatching and finally growth as well as development* 

 becomes completely inhibited. Thymus does not contain either specific 

 growth-promoting nor specific development-inhibiting substances; the in- 

 hibition of growth and development caused by poire thymus diet is a 

 deficiency phenomenon and must be due to the fact that substances in- 

 dispensable for the processes of growth and development are lacking in 

 the thymus. 



