STUDIES ON SEX-DETERMINATION IN AMPHIBIANS. 35 



them dying in the early stages of their development. Of the 

 thousand or more eggs with which the experiment started only 

 372 tadpoles lived until it was possible to ascertain their sex, and 

 in fully one half of this number the gonads had to be sectioned 

 in order to make sure of the sex. Of the 372 individuals in 

 which sex was ascertained 178 were males and 194, or 52.15 per 

 cent., were females. The sex ratio found in these individuals 

 which developed from "overripe" eggs agrees essentially with 

 that found among the individuals in the first experiment where 

 presumably the eggs were in very early phases of ripening at the 

 time that they were fertilized. In both of these experiments an 

 excess of females was produced, although, to accord with Hert- 

 wig's theory, there should have been an excess of males in each 

 case. 



The results ol these experiments seem to indicate that in Bufo 

 sex is not determined by the ripeness of the egg at the time that 

 it is fertilized. Eggs fertilized normally within a period of four 

 hours or artificially seven hours after the death of the female, 

 eggs laid the last of March as well as those laid near the end of 

 April, all produce a slight excess of females and thus give a sex 

 ratio that is practically the same as that found among young toads 

 that have recently completed their metamorphosis under natural 

 conditions. 



In connection with these experiments reference may be made 

 to a theory of sex-determination recently elaborated by a physi- 

 cian, Dr. T. E. Reed ('07). Starting with the supposition that the 

 ovum is hermaphroditic and that sex, which is a property or 

 function of the ovum, is determined at the time of the fertilization 

 of the egg, Reed then assumes that there is a sex cycle in the 

 germ-plasm and that "this rhythm extends over a period of 

 twelve hours, six being active or masculine, and six passive or 

 feminine, and changes from day to day as do the tides. . . . 

 The sex of the embryo is determined simply by the period through 

 which the germ plasm happens to be passing when fertilization 

 takes place." According to this theory if an egg is fertilized 

 during the positive period, i. e., when the*tjde is rising, the resultant 

 embryo is always a male ; if fertilization takes place during the 

 negative period, when the tide is ebbing, the embryo is neces- 



