STUDIES ON SEX-DETERMINATION IN AMPHIBIANS. 



IV. THE EFFECTS OF EXTERNAL FACTORS, ACTING BEFORE 

 OR DURING THE TIME OF FERTILIZATION, ON THE SEX 

 RATIO OF Bufo lentiginosus. 



HELEN DEAN KING, 

 THE WISTAR INSTITUTE. 



At the present time practically all of the investigators who are 

 working on the problem of sex-determination believe that the 

 sex of a zygote is fixed at a very early period, and that it is not 

 dependent upon environmental factors acting during embryonic 

 development. There is considerable diversity of opinion, 

 however, regarding the time when sexual differentiation takes 

 place and the way in which it is brought about. 



A number of investigators, among whom may be mentioned 

 Rauber ('oo), Beard ('02), Schultze ('03) and Russo ('09) main- 

 tain that sex is determined in the ovary, chiefly by nutritive 

 conditions. According to this view mature eggs are either male 

 or female, and the spermatozoan can take no part whatever in 

 determining the sex of the egg it fertilizes. 



Other investigators, prominent among whom are Stevens ('05), 

 Wilson ('06) and Morgan ('07, '09, '10), are inclined to the opinion 

 that sex is not determined until the time of fertilization, and that 

 it is the spermatozoan that definitely and unalterably fixes the 

 sex of the individual. 



These two views are apparently irreconcilable, and such an 

 array of seemingly indisputable facts has been brought forward 

 in support of each of them that it may be, as Jordan ('09) has 

 recently suggested, that "sex has been attained by several paths, 

 and is now determined in different modes and at different times 

 in the different groups of animals and plants." If this be true, 

 it will be futile to try to find a single factor, or set of factors, 

 that is the fundamental cause of sex in all organisms ; and in- 

 vestigations in this field must be confined to an attempt to dis- 



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