Witschi and McCurdy 665 



unpublished experiments prove that even loss of tlie hypophyses by both 

 twins does not change the larval sex antagonism. 



Having therefore made sure that we deal actually with gonadal antago- 

 nism, it becomes immediately evident that not only do the sex glands of 

 Triturus respond to the gonadal substances of the Ambyslomas, but that thcv 

 serve as extremely sensitive indicators. This is in keeping with the previously 

 established rule that heterosexual parabiosis produces more extensive reac- 

 tions in the gonochoristic species and races than in those with hermaphrodite 

 tendencies (Witschi*). Sensitivity grows from A. maculatum to A. jefferson- 

 ianum, A. tigrinum, and T. torosus. It fits well with this rule that maculatum 

 males and females reduce the gonads of Triturus more than those of twins of 

 their own kind; or that the female of A. tigrinum, which manifests a domi- 

 nance over male salamanders only under special conditions of time and size 

 (Witschi"), is able to completely suppress testicular differentiation in Triturus 

 males. 



Yet there still remains the problem of unisexual antagonism which is ap- 

 parent in the suppression of Triturus ovaries by Ambystoma ovaries and of 

 Triturus testes by Ambystoma testes.* Of course, it is quite probable that 

 every larval gonad releases simultaneously corticin as well as medullarin, 

 albeit in different proportions. At least for all forms which, like the sala- 

 manders, are strongly inclined toward hermaphrodism, it is safe to make this 

 surmise. All differentiating gonads consist morphologically of double pri- 

 mordia: cortex and medulla. Sex differentiation consists, in morphological 

 terms, in the further development of one and the relative reduction and sup- 

 pression of the other. It can, therefore, be taken for granted that in female 

 pairs a small amount of medullarin, released by the Ambystoma ovary, is 

 retarding ovary formation in Triturus, and likewise in male pairs, that corticin 

 given off by the salamander testes holds back the development of male gonads 

 in a newt co-twin. In view of the previously established high responsiveness 

 of Triturus gonads, this new aspect of the ambisexual nature of vertebrate 

 sex glands certainly must be given consideration as a factor in the interpreta- 

 tion of unisexual gonadal antagonism. Yet it cannot in itself serve as a com- 

 plete and satisfactory explanation, since there still remains the odd fact that 

 in the maculatum-Triturus combinations, the Triturus testes are more in- 

 hibited by salamander testes than by salamander ovaries (table i). It certainly 

 seems unlikely that male salamanders produce more corticin than females 

 of the same species. Therefore, as in an earlier discussion of conditions ob- 

 served in heterogenous frog twins (Witschi"), w'e feel again impelled to assume 

 that the growth of certain organs, and especially the sex glands, is controlled 

 by special growth substances which are available only in limited quantity. 

 They are possibly responsible for compensatory hypertrophy after partial 

 castration, as well as for the often-experienced difficulty of establishing suc- 



* According to a recent paper by Humphre)" similar conditions occur in A. maculatum 

 hosts carrying large gonads derived from implanted A. ligriyium preprimordia. 



