2O6 HELEN DEAN KING. 



cover the conditions that determine sex in the particular form 

 selected for study. 



The important experiments of Tower ('10) with the eggs of 

 the potato-beetle, Leptinotarsa, and also those of Tennent ('10) 

 with cross-fertilized echinoderm eggs, have shown that very 

 definite alterations in the structure of the developing organism 

 can be produced by changing the external conditions that sur- 

 round the egg at the time of fertilization. The recent investi- 

 gations of Shull ('ioa, '106) and those of Whitney ('10) on the 

 rotifer, Hydatina senta, indicate that the chemical composition 

 of the water in which the cultures live determines the appear- 

 ance of the sexual cycle. But what the substances are that 

 bring about the appearance of the sexual forms, and how they 

 act, has not yet been ascertained. 



All of these experiments show that environmental factors 

 can greatly change the course of development even if they act 

 for only a comparatively short period on the unsegmented egg: 

 whether they can influence anything as fundamental as sex, 

 however, remains to be determined. It was for the purpose 

 of testing this point that a series of experiments on the eggs of 

 the toad, Biifo lentiginosus, was undertaken two years ago. 

 The first of these experiments showed pretty conclusively that 

 temperature, acting at the time of fertilization is not the domi- 

 nant factor in the determination of sex in this form (King, '10). 

 The present paper records the results of the second series of 

 experiments along this line. It is hoped that the method of 

 investigation employed will eventually settle the point as to 

 whether sex in Bufo is determined at or before the time of fer- 

 tilization, even if it gives no clue to the factors that are involved 

 in this process. 



The Wistar Institute of Anatomy and Biology has provided 

 an equipment for the carrying on of these investigations that is 

 nearly ideal for the purpose. This equipment consists of two 

 iron racks, each of which supports three tiers of twenty galvanized 

 iron trays 9X12X23/2 inches: dishes of this size are convenient 

 for handling and each gives ample space for fifty large tadpoles. 

 A series of %-inch iron pipes, connected with the city water 

 system, are attached to the supporting frame five inches above 



