CHAPTER III. 



The Determination of Sex (^Hypotheses and Observations). 



So far the differences between the sexes as observed in adult 

 forms. Attention must now be turned to the origin of sex itself 

 in the individual organism. The historic beginning of sex will 

 be discussed at a later stage ; the present problem concerns 

 the factors which determine whether any given organism will 

 develop into a male or into a female. The question, in other 

 words, is that usually known as the determination of sex. 



§ I. The Period at wiiich the Sex is Determined. — Every 

 organism, whether male or female, develops from a fertilised 

 egg-cell, apart of course from the occurrence of asexual and 

 parthenogenetic reproduction. This material, which in one 

 case develops into a male, in another into a female, is, so far 

 as our experience can go, always the same ; and when the sex of 

 the organism is absolutely decided, is a question to which no 

 general answer can be given. In the higher animals (birds 

 and mammals) it is possible at quite an early date in embryonic 

 life to tell whether the young organism will turn into a niale or 

 a female, though in the very earliest stages it is impossible to 

 determine whether the rudiment of the reproductive organs is 

 going to become a testis or an ovary. But in lower vertebrates, 

 such as frogs, the period of embryonic indifference is greatly 

 prolonged ; and it seems certain that a hatched tadpole, even 

 after a tendency towards, say maleness, has actually arisen, 

 may in certain conditions have this altered in the opposite 

 direction. Among invertebrates, the sexual organs are often 

 late in acquiring definite predominance in favour of either sex, — 

 that is, the period of undecided indifference is, as one would 

 expect, usually much longer. 



The factors which are influential in determining sex are 

 numerous, and come into play at different periods, so that it 

 is quite possible for a germ-cell to have its future fate more 

 than once changed. The constitution of the mother, the 

 nutrition of the ova, the constitution of the father, the state of 



