Witschi and McCurdy 659 



hermaphrodite features are observed with striking regularity (Gilbert""), most 

 frequently in the Ozark race of A. maculaturn, which is represented in seven- 

 teen of the nineteen maciilatum-Triturus pairs (table i). 



Besides the single normals, the homogenous parabiotic pairs of the various 

 species must also be considered as controls of the experimental heterogenous 

 twins. In Triturus torosus, the sex glands of female-female and male-male 

 twins develop normally, like those of single controls. In female-male com- 

 binations, one observes a severe antagonism from which, at the beginning, 

 both sexes suffer almost equally. It becomes evident even before morphologi- 

 cal sex differentiation, and interferes with the formation of the indifferent 

 gonad primordia. Later, the male recovers normalcy while the female be- 

 comes nearly or completely sterile (Witschi and McCurdy''; detailed paper in 

 preparation). In heterosexual pairs of salamanders, the antagonism is less 

 pronounced, and only ovarial development suffers appreciably. The Ozark 

 race of ^. maculatiim is especially notable for a very low degree of interference 

 between members of heterosexual pairs (Witschi,* " the latter contains refer- 

 ences to the important papers of Burns and Humphrey). 



Experimental Series 



The Californian newt, Triturus torosus (family Salamandridae), was joined 

 with each of the three mentioned species of the genus Ambystoma (family 

 Ambystojnidae). The three combinations will be designated by the symbols 

 mac-T, tig-T and jef-T. Embryos of the tailbud stage were grafted together 

 by the previously described routine technique (fig. 4). In early experiments, 

 one embryo was joined with its right side to the left of another (figs. 5 and 7); 

 later it was found to be of some advantage to use the right side in both em- 

 bryos for grafting (figs. 4 and 6). At first, no unusual difficulties appear to exist. 

 Healing and recovery from the operation are as easy as in homogenous twins. 

 Later on, however, the salamander remains immotile for two or three weeks, 

 due to a poisonous stibstance contained in the newt (Twitty"). This paralysis 

 of the salamander during the critical period when feeding habits normally 

 become established, is probably the main reason for the excessive mortality 

 in this experimental group. Of about eight hundred pairs of embryos which 

 were successfully joined, only thirty-nine survived until the time of metamor- 

 phosis or longer. Anastomoses between large blood vessels become established 

 very early (fig. 5), and even at the metamorphosis stage, blood is still freely 

 exchanged. The largest pair was preserved at an age of thirteen and one-half 

 months; it had passed through metamorphosis when six months old. Since the 

 change from the acjuatic to the terrestrial life proved to be a great hazard, most 

 pairs were preserved toward th.e end of the metamorphic period. As a rule, the 

 newt is the one of the pair which accepts food first. Later, after recovery from 

 the effect of the poison, the salamander also begins to eat and sometimes even 

 assumes the leading role both in swimming and eating. Like normal sala- 

 manders and newts, most twin pairs metamorphose at an age of from four to 



