138 EMIL WITSCHI. 



chick gonads, shortly before or after their sexual differentiation, 

 into the chorio-allantoic membrane of nine-day-old host embryos. 

 The material was preserved nine days after the operation. In 

 no case of heterosexual combinations was there found a specific 

 modification in the process of sex-differentiation of the graft, 

 nor any trace of sex-reversal in the gonads of the host embryo. 



Briefly, we can say that the experiments with mammals and 

 birds failed to show a definite indication of a tendency towards 

 a physiological sex-reversal. 



On the other hand Burns (1925) reported a very successful 

 experiment on the urodele Amblystoma. He joined young 

 embryos during the tail-bud stage in parabiosis. Instead of the 

 expected chance combinations of the sexes, requiring a ratio of 

 i cf cf : i cf 9 : i 9 c? : i 9 9 , he obtained exclusively one-sexed 

 pairs. 1 Burns is inclined to interpret the numerical result of 

 44cfcT + 369 9 as a i : i ratio and infers, that in about one 

 half of the original cf 9 and 9 cf combinations the males 

 changed into females, while in the other half, the females changed 

 into males. He claims, that "there is no prepotency, which 

 constantly favors a given sex." But he did not happen to see 

 any developmental stage of the supposed sex- transformation. 

 He finds the pairs to be one-sexed already in the very earliest 

 phases of sex-differentiation. Therefore the essential part of 

 the process remains obscure. Burns' experiments, although 

 successful in showing that a transformation of sex occurs, did 

 not solve the problem of how it occurs. 



In the spring of 1926 the writer started new experiments in 

 which he joined young frog embryos in parabiosis. The operation 

 was performed shortly after the closure of the medullary tube; 

 that is, at an age of approximately 50 to 70 hours, when the eggs 

 were kept in the laboratory at room temperature (i8-2O C.). 

 In normal controls the first sex-differentiation takes place during 

 the third week. In the parabiotic twins it is often somewhat 

 delayed. The twins were preserved at intervals during the larval 

 period and the stage of metamorphosis (Fig. i). 



The material belonged to four different species, but in the 

 following we consider only the experiments with Rana sylvatica, 

 the American wood-frog. 



1 cf ? means male on the left, female of the right side; and vice versa. 



