66o 



Sex in Parabiotic Twins 



six months (room temperature); though poor feeders may be delayed, some as 

 much as two or three months. 



In only one of the thirty-nine pairs were both mates of approximately nor- 

 mal size at the time of metamorphosis (jef-T^; fig. 7). Quite obviously, there 

 exists an imbalance in these twin systems, tending toward dwarfing or even 

 elimination of one of the partners. Surprisingly enough, Triturus is the loser 

 in thisstruggle when combined with ^. maculatum (fig. 1), but the winner here 

 in competition with the normally larger A. jeffersonianum and A. tigrinum 

 (figs. 2, 3). In considering the measurements given in tables 1-3, one must take 

 into account the fact that the normal size of tiger salamanders at the meta- 



J 



Fig. 1. Heterogenous parabiotic pair, Ambystoma maculatum x Triturus torosus, at the 

 end of the metamorphosis stage. The salamander (white) is larger and the newt (stippled) 

 smaller than normal size. Four-sevenths actual life-size. 



Fig. 2. Heterogenous pair jef-Tj^, at a late metamorphosis stage. The salamander is of 

 pigmy size, but its testes are of about normal dimensions (the folded organ shown at the 

 right side). The newt is rather larger than normal, but its gonads are reduced to narrow 

 vestiges (center figure). Twins are four-sevenths life-size, sex glands x 6. 



Fig. 3. Heterogenous pair tig-T,, at a late metamorphosis stage. The dwarfed salamander 

 (stippled) has ovaries of normal size and structure (left figure). The newt has undifferentiated 

 gonads of rudimentary size (center figure). Twins are four-sevenths life-size, sex glands x 6. 



morphosis stage is from about 140 to 180 mm. Even the case tig-Tg (table 3) is, 

 therefore, no exception from our generalization, for the newt of 50 mm. is 

 above the average size of controls, while the salamander of 90 mm. is under- 

 sized. The reasons for this peculiar growth phenomenon are not clear, but it is 

 significant that the developmental rates of the sex glands do not follow the 

 same trends as the soma. 



The excessive mortality rate seems to influence the initial sex ratio but 

 very slightly. Adding up all surviving salamanders, we find twenty-two females 

 and fourteen males. The moderate prevalence of females is in line with pre- 

 vious similar observations on somewhat greater vitality of this sex in parabiotic 



