THE FACTORS WHICH DETERMINE SEX 663 



more recent researches, for Gurnet's experiments/ conducted on 

 similar lines to those of Born and Yung, show a preponderance 

 of males among tadpoles which were fed upon animal food, and 

 an approximate numerical equality among those which received an 

 exclusively vegetable diet. Moreover, the method adopted by Born 

 for ascertaining the sex of the individual tadpoles during the period 

 of metamorphosis seems to have been unsatisfactory, since it was 

 based on the assumption that the ovary is always larger than the 

 testicle, whereas this is not invariably the case. It is stated also 

 that frogs' eggs from certain localities yield a higher percentage of 

 females than those from other localities, and consequently that a 

 disproportion of the sexes may exist under normal conditions ; but 

 this fact in itself does not show that sex is not determined by 

 nutritive or other environmental influences, but may point to a 

 directly opposite conclusion. But, as Morgan points out, if the 

 natural disproportion between the two sexes is great, errors may 

 easily creep into the experimental results.^ Again, King's observa- 

 tions relating to sex-determination in Amphibians provide no 

 evidence that either food or temperature are. factors in this process.^ 



Since the early work of Born and Yung, however, many other 

 observations of a more critical character have been made on the 

 question of sex-determination in Amphibians. The most remarkable 

 of these has been the recent work of Witsehi.* A full summary of 

 these observations down to the latter part of 1921 will be found in 

 Crew's ^ paper. It would seem that in the frog while the chromo- 

 some constitution may determine sex at fertilisation, in §ome 

 instances this is clearly overridden during subsequent development, 

 which results in the production of " somatic " males or masculinised 

 females. The mechanism by which the female is transformed is 

 one which acts through the internal secretions of the gonads. 



Thus it would seem that in Amphibians sex is not definitely 



' Cui^not, " Sur la Determination dii Sexe chez les Animaux," Bull. Sci. de 

 France et Belg., vol. xxxii., 1899. 



2 Morgan, Experimental Zoology, New York, 1907. 



^ King, "Food as a Factor in tlie Determination of Sex in Amphibians," 

 Biol. Bull., vol. xvi., 1909. "Temperature as a Factor, etc.," Biol. Bull., 

 vol. xviii., 1910. See below, pp. 668-669 and pp. 692-693. 



"• Witschi, " Der Hermaphrodismus der Frosches und seine Bedetitung fur 

 das Geschlechtsproblem und die Lehre von der inneren Sekretion der 

 Keimdriisen," Arch. f. Entwick., vol. xlix., 1921, also short paper in Am. Nat., 

 1921. Witschi describes the frog as having an indifferent gland which might 

 develop either into an ovary or into a testis, but the latter is rare. Far more 

 individuals develop first ovaries which are later transformed into testes in the 

 larva. Oviducts grow in association with ovaries up to the time of transforma- 

 tion. Lateral hermaphrodites are not uncommon. (See footnote, p. 700.) 



^ Crew, " Sex-Eeversal in Frogs and Toads : A Eeview of the Eecorded 

 Oases of Abnormality of the Eeproductive System and an Account of a 

 Breeding Experiment," Jow. of Qen., vol. xi., 1921. 



