SEX DIFFERENTIATION IN HETEROGENOUS 

 PARABIOTIC TWINS (AmbystomaxTnturus)* 



WITH growing information on sex development in parabiotic twins, definite 

 indications of taxonomic specificity of the sex-differentiating substances 

 are increasingly demanding our interest. It has been found (Witschi^'*) that 

 in some species these substances, corticm and medullarin, are spread by the 

 blood stream like hormones, wdiile in others they diffuse more slowly and 

 with falling concentrations through the tissues. In at least one amphibian 

 species, their presence was noticeable even before the initiation of morphologi- 

 cal sex differentiation (Witschi and McCurdy^). On the contrary, in all other 

 well-analyzed cases, the antagonism between the sexes appears only at later 

 developmental stages (Witschi,^* ^comprehensive review®). 



This variety of conditions immediately suggests further investigations by 

 the method of heterogenous parabiosis. It would seem interesting indeed to 

 know what effects might be obtained from inductive substances of sala- 

 manders, poured into the blood streams of frogs. Unfortunately, the possibili- 

 ties of such testing are limited by the factors of compatibility in heteroplastic 

 grafting. In our laboratory, salamander-frog and newt-frog twins lived for 

 several weeks, but they died invariably before the stage of initial sex differen- 

 tiation. More successful were the experiments with salamander-newt twins. 

 The fact that grafting between taxonomic orders (Caudata and Salieritia) and 

 families (Salamandridae and Anihystoynidae) yields twin combinations which 

 may last for weeks and even years is in itself remarkable, and deserves further 

 study; the present paper will, however, be restricted to the analysis of the 

 interactions observed in the sex development of the heterogenous twins. 



Controls 



In these experiments, three species of salamander, Ambystoma maculatiim, 

 A. jeffersonianum^ and A. tigrmum, were used in combination with the Cali- 

 fornian newt, Triturus torosiis. The latter is smaller than Ambystoma tigri- 

 num, but slightly larger than A. m,aculatiim. This is true particularly for the 

 late larval and metamorphosing stages, while adult newts and spotted sala- 

 manders are nearly of same size. A. jeffersonianiwi in Iowa is represented by 

 a series of size variations. A dwarf form from Cedar Falls has been described 

 in 1937.^ The typical form is larger than T. torosiis and A. macxilatum, but 



* The first cxpeiiments ol this series were started in 1927 while, as a fellow of the Rocke- 

 feller Foundation, the senior author spent three months in the Anatomy Department of the 

 University of California. In spite of the length of the interval, the aiUhors are desirous of 

 expressing their gratitude for the cordial hospitality extended to them hy Dr. Herbert M. 

 Evans and for the stimulating interest and the generous help with \vliich he furthered their 

 work. During several years the continuation of the experiments was made possible by grants 

 from the National Research Council, Cominittee for Research in Problems of Sex. 



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