144 Transactions of the Society, 



Two Preliminary Cautions. 



5. Two preliminary cautions may be noted. In the first place, 

 these main theories are not in any very strict way mutually exclu- 

 sive. Even if we conclude that there are two kinds of ova in the 

 ovary, one set pre-determined to develop into males and the other 

 set pre-determined to develop into females, it does not follow that 

 the relative numbers of these may not be changed as life goes on, 

 e.g. in relation to the diet of the parent. Even if we conclude that 

 there are two kinds of eggs, it does not follow that the determina- 

 tion of these is absolute ; it may be strengthened or weakened in 

 the process of fertilization, or by environmental influences during 

 early development. 



In the second place, we must be careful in arguing from one 

 set of organisms to another. 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 quite well be 

 that sex is determined by a variety of factors operative in different 

 cases and at different stages in development. 



a. — do environmental conditions, operating on the sexually 

 undetermined developing offspring-organism (after 

 fertilization), share in Determining its Sex? 



6. The first possibility which we shall discuss is that environ- 

 mental influences acting on the developing organism (embryo or 

 larva), may determine or help to determine whether it is to become 

 a male or a female. This possibility assumes that in certain cases 

 the developing organism is for some time indeterminate as regards 

 sex. That this is not a wild assumption, is evident when we 

 remember that in the case of many animals it is for a long time 

 impossible to distinguish the sexes. 



In support of the theory that environmental influences may 

 determine sex in developing organisms, assumed to be indetermi- 

 nate, reference has been made for many years past to a series of 

 experiments on tadpoles, carried on by Professor Emile Yung, of 

 Geneva, to whom recognition is due for beginning experimental 

 investigation of the subject at a time when that mode of approach 

 was much neglected. 



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

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

 aphroditism. 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 mor- 

 tality, 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 



