THE HORMONE THEORY 165 



seasons caused the pad to remain in the condition in 

 wliich it was at the time, that there was no reduction 

 or absorption as Nussbaum and Meisenheimer found, 

 and that allo-transplantation of testes — that is, the 

 introduction of testes from other frogs either into the 

 dorsal lymph-sacs or into the abdominal cavity — or 

 the injection of testis extract, had no effect in 

 causing growth or development of the thumb-pad. 



There seems to be one defect in the papers of both 

 Nussbaum and Smith and Schuster — namely, that 

 neither of them mentions or apparently appreciates 

 the fact that the thumb-pads, apart from the dermal 

 glands, consist of horny epidermis developed from 

 the living epidermis beneath. The horny layer is 

 not shown clearly in the figures of Smith and Schuster. 

 It seems impossible that the horny layer or its 

 papillae could atrophy in consequence of castration, 

 or be absorbed. The horny part of the frog's 

 thumb-pad is comparable with the horny sheath of 

 the horns in the mammalian Prong-buck (Aiitilocajyra) 

 which are shed after the breeding season and annually 

 redeveloped. Meisenheimer claims that he produced 

 development of papillae on the thumb-pad, not only 

 by implantation of pieces of testis, but also by im- 

 plantation of pieces of ovary. This seems so very 

 improbable that it suggests a doubt whether the 

 same investigator was not mistaken with regard to 

 the results of his experiments in transplanting 

 gonads in Moths. 



Smith and Schuster conclude that the normal 

 development of the thumb-pad depends on the 

 presence of normal testes, but that there is no sulli- 

 cient evidence that the effect is due to a hormone 

 derived from the testis. It is equally probable, 



