REPRODUCTION AND SEX 523 



Even if we conclude that there are, for instance, two kinds of ova 

 in the ovary, one set predestined to develop into males and the 

 other set predestined to develop into females, it does not follow 

 that the relative numbers of these cannot be changed as life goes on, 

 e.g. by the diet of the parent. And even if we conclude that there are 

 two kinds of ova predestined from the start, it does not follow that 

 the predestination need be quite unalterable by the conditions of 

 fertilisation and of development. 



Another preliminary caution must be noted. What determines 

 sex in frogs may not hold true for cattle; what determines sex in 

 Rotifers may not apply to birds. Nature is very manifold, and it 

 may be that sex is determined by a variety of factors operative in 

 different cases and at different stages. 



(a) FIRST THEORY,— That environmental influences, operating 

 on the sexually undetermined offspring (after fertilisation), may have 

 at least a share in determining the sex. 



In many young organisms it is for a time impossible to dis- 

 tinguish the sexes, and the assumption is often made that there is 

 a prolonged indeterminateness as regards sex. So the first theory 

 that we need discuss is, as stated above, that environmental influ- 

 ences give the bias towards maleness or femaleness. 



In support of this theory it has been customary to refer to the 

 interesting experiments on tadpoles made by Professor Emile 

 Yung, of Geneva, who began experimental investigation of the 

 subject at a time when this mode of approach was little thought of. 



Let us recall some of Yung's evidence. Tadpoles are said to 

 linger for some time in a state of sex-indifference or potential her- 

 maphroditism. In normal conditions there are about 57 females to 

 43 males in the hundred. But tadpoles fed with beef, fish, and frog- 

 flesh yielded respectively 78, 81, and 92 females in a hundred. This 

 was, of course, a very interesting result, but it has been pointed 

 out that Yung did not pay sufficient attention to differential 

 mortality, that he had not sufficiently large numbers, and that 

 although some tadpoles are potentially hermaphrodite (with testes 

 around the ovaries), there are others which are quite distinctly male 

 or female even in young stages. 



When caterpillars are underfed, there is an unusually large 

 proportion of males (Landois, Treat, Gentry, and others). But as it 

 was shown long ago that the sex of the caterpillar or of the future 

 adult is determined before the larva leaves the egg, starving experi- 

 ments are irrelevant. They only show that there may be great 

 differences in the rate of juvenile mortality in the two sexes. 



Nor is there agreement among the results of experiment. Kellogg 

 and Bell found that the sex of the silkworm is not appreciably 

 affected by the nutrition of the parents or even of the grandparents 



