Witschi and McCurdy 663 



in the combinations with jeffersoniaruim or iigriniirn salamanders. As a rule, 

 they consist only of long, nanow, and threadlike folds (figs. 2 and '^) which, 

 on sectioning, prove to contain but few primordial gonia (fig. 12T) and to be 

 sexually undifferentiated. In the tables, the symbol i (indifferent) is followed 

 by / or 7)1 if a majority of the germ cells are found in either the cortical (as in 

 fig. ST), or the medullary (fig. 12T) position. This relationship is not suf- 

 ficient, however, to establish with certainty the basic sex; for in normal newts, 

 at the indifferent stage, the gonia are also scattered throughout the gonad 

 primordia. 



In jef-T^, the newt glands are definitely on the way to ovarial differentia- 

 tion, a few germ cells having become transformed into synaptic ovocytes. 

 Similarly, in jef-T,., a nimiber of gonia are clearly located in medullary cords 

 and some segments of the glands have a distinctly testicidar appearance. In 

 one pair, the newt gonads are nearly normally developed. This is the pre- 

 viously described case of jef-T^ (figs. 7, 11), in which the salamander has fairly 

 large ovaries and the newt good-sized testes. These three cases form an im- 

 portant link between the first and the two other series, for in the seven re- 

 maining jef-T and in all tig-T pairs the newt gonads are reduced to diminutive 

 primordial rudiments, while the salamander gonads are of the impressive 

 size (figs. 2, 3 and 12), which is characteristic for normal specimens of these 



two species. 



Discussion 



The present series of heterogenous twins differs markedly from the many 

 previously described groups of homogenous twins or combinations of various 

 races or species of salamander, especially in the following two features. 



1. Tritiirus does not interfere with normal gonad development of Am- 

 bystoina co-twins, in any of the four possible sex combinations. 



2. Amhystomas of the three species inhibit the development of co-twin 

 gonads not only if the newt is of opposite sex, but also if it is of same sex. 



The first result is very smprising in view of earlier work which had given 

 proof that the newt is a distinctly gonochoristic species and that its gonads 

 emit morphogenic substances of the corticin and medullarin type (Witschi 

 and McCurdy''). As stated before, in heterosexual pairs of newts (TJ-TJ*), 

 the severe mutual antagonism becomes manifest even before morphological 

 sex differentiation, that is, at so early a stage, that the failure of the same 

 species to produce effects when in parabiosis with salamanders cannot be at- 

 tributed to a time factor. The lack of effectiveness of the newt substances is 

 drastically demonstrated by the five tnaculatum^-Trituriis^ pairs (fig. 9), 

 in which large testes of Tri turns coexist with large and absolutely normal 

 Ambystoma ovaries. In Triturus^-Tritiirusi^ pairs, testes of ecjual size and 

 normalcy reduce the female gonads to near sterile, and often entirely undif- 

 ferentiated, rudiments. It must, therefore, be concluded that the inductive 

 sex substances of Tritiirus torosus are of a specific nature which prevents them 

 from becoming effective in the salamander co-twins. Since blood is exchanged 



