Witschi and McCurdy 667 



2. Both sexes of the salamander produce corticin as well as niedullarin— 

 though at different ratios— and suppress thereby ovarial as well as testicular 

 development of newt co-twins. 



3. In order to understand the reactions in unisexual pairs, one is forced to 

 assume that gonad size is limited by special ovarial and testicular growth 

 stibstances. These substances seem to be utilized and used up more quickly 

 by tlie salamanders than by the newts. 



The unequal growth of the gonads in heterogenous twins is not a unic^ue 

 phenomenon. In the above-mentioned frog twins (R. sylvalica x R. tempor- 

 aria), it was found that the spleen of the sylvatica member is always greatly 

 enlarged, while in the temporaria it is reduced to a mere vestige. In the present 

 experimental series, we find that in general body growth, Ambystoma macu- 

 latum surpasses Triturus, while the other two salamander species lag behind 

 their newt co-twins, especially during the late larval stages. Of great interest 

 is the development of the kidneys. In the mac-T combinations, the kidneys 

 are somewhat larger and better differentiated in the maciilatum than the 

 Triturus member. In all jef-T and tig-T pairs, however, the Triturus kidneys 

 alone are normally developed, while the salamander kidneys are inhibited, 

 of small size and histologically incompletely differentiated. There is no cor- 

 relation between size of the kidney and size of the sex glands. Kidney develop- 

 ment and body size run somewhat parallel, though in jef-T^, the salamander 

 is of normal size while its kidneys are greatly inhibited. 



Summary 



1. Of about eight hundred pairs of salamander-newt twins which were tuiited 

 in parabiosis at the tailbud stage, only tliirty-nine lived to tlie metamorpliosis 

 or later stages. 



2. In heterogenous pairs, growth of the twins is usually uneven: so that one 

 member is of normal size, or even larger, while the other member is dwarfed. 



3. Similarly, the kidneys of the two species develop differently. 



4. It is stiggested that the growth of various organs is controlled by specific 

 growth substances, which are produced in limited amotmts. In heterogenous 

 parabiosis, one twin may use up more than its share of such substances. 



5. With respect to sex, the three species of salamanders used in this experi- 

 ment are of markedly different type. Ambystoma maciilatum shows distinct 

 hermaphrodite tendencies (ss). A. jeffersonianiim is of slightly hermaphrodite 

 (s) or prevailingly gonochoristic (d) type. A. tigrinum is prevailingly gono- 

 choristic (d). The newt. Triturus torosus, with which these salamanders Avere 

 combined, is of distinctly gonochoristic type (del). 



6. Sex development in salamanders is not modified by parabiosis with newts. 

 It is concluded that the sex-differentiating substances, corticin and medul- 

 larin, have a certain taxonomic specificity and that salamander gonads do not 

 respond to the substances emitted by the newts. 



