136 HELEN DEAN KING. 



A comparison of the figures given in Table II. with those of 

 Table III. shows that there is not the uniformity in the results of 

 these various series of experiments that one would expect to find 

 if temperature alone determined sex at the time of the fertilization 

 of the egg. A lot of eggs that was fertilized at a temperature of 

 26° C. gave a higher percentage of females than was obtained 

 from a lot that was fertilized at a temperature of 35° C, while 

 the highest percentage of females obtained as yet in any experi- 

 ment (62.5 per cent.) was found in a lot of individuals that de- 

 veloped from eggs that were fertilized at a temperature of 16- 

 18° C. Of the various series of eggs that were fertilized in cold 

 water, those that were fertilized at 9° C. gave a much lower 

 percentage of females than did other lots that were fertilized at 

 10° C. or at 5° C. Although these results show very conclu- 

 sively that temperature, acting at the time of the fertilization 01 

 the egg, is not the dominant factor in the determination of sex 

 in Bufo, they do not exclude the possibility that temperature may 

 have an indirect action on the determination of sex in this form. 

 In every series of experiments lots of eggs that were fertilized at 

 a temperature of 13° C. or below have given a lower percentage 

 of females than has been obtained in the individuals developing 

 from lots of eggs that were fertilized at higher temperatures. It 

 is conceivable that a low temperature might act more injuriously 

 on the female-producing spermatozoa than on those that are 

 male-producing, if it be that there is a dimorphism in the sperma- 

 tozoa of B2(fo and that the male determines sex. This would, 

 of course, greatly increase the chances that an egg laid in cold 

 water would be fertilized by a male-producing spermatozoon. 

 If, on the other hand, sex is determined in the egg, it is possible 

 that the sex-determining mechanism is so evenly adjusted that 

 temperature, under certain conditions, may turn the scale in one 

 direction or the other. 



LITERATURE. 



1. Hertwig, R. 



'05 iJber das Problem der sexuellen Differenzierung. Verhandl. Deutsch. Zool. 

 Gesellschaft, 1905. 



2. Issakowitsch, Alex. 



'06 Geschlechtsbestiramende Ursaclien bei den Daphniden. Arch. mikr. Anat., 

 Bd. LXIX., 1906. 



