ACTIVATORS, KINASES AND HORMONES 193 



it is dependent upon influences other than those arising 

 from functional use. What these influences may be is at 

 present a matter of pure speculation. In his recent most 

 interesting contributions to our knowledge of growth Rub- 

 ner has been led to assume that the property of growth in 

 the young organism is connected with certain special chem- 

 ical complexes in the protoplasmic material, complexes 

 which have nothing directly to do with the simple main- 

 tenance of the nutrition of the cell and which after adult 

 life is reached disappear for the most part from the general 

 soma. In line with this hypothesis one might assume that 

 the hormones given to the blood by the reproductive cells 

 contain such complexes which when anchored in certain 

 tissues lead to an accelerated growth. Perhaps the clearest 

 and most interesting experiments made upon the reproduc- 

 tive hormones are those reported by Nussbaum. He chose 

 for his experiments the males of Rana fusca whose repro- 

 ductive organs go through a cyclical period each year. At 

 the proper period the preparation for the mating season 

 shows itself in the hypertrophy of the seminal vesicles, of 

 the thumb pads, and of certain muscles in the forearm. If 

 the frog is castrated these hypertrophies do not occur, or 

 if they have begun before the castration is performed retro- 

 gressive changes take place. On the other hand, the usual 

 hypertrophy of the nuptial organs can be initiated in a 

 castrated frog if pieces of the testis from another frog are 

 introduced into the dorsal lymph sacs. The pieces thus 

 introduced do not become grafted permanently, but are 

 gradually absorbed and the growth of the thumb pads and 

 of the muscles in the forearms falls off after this absorption 

 is completed. Nussbaum believes that the stimulating effect 

 of the testicular hormones is not exerted directly upon the 

 tissues which show the increased growth, but rather upon 

 the portions of the central nervous system which innervate 

 these tissues. This belief rests upon the experimental fact 

 that if the peripheral nerves going to the glands and pa- 

 pillae of the thumb pads are severed on one side the testic- 

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