THE DETERMINATION OE SEX. 27 



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CHAPTER III. 



THE DETERMINATION OF SEX. 



{Hypotheses mid Observations. ) 



Co 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. 



I. The Period at which the Sex is Determined. — Every 

 organism, whether male or female, develops from a fertilized egg-cell, 

 apart of course from the occurrence of a sexual 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 

 male 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 hatch xi tadpole, even after a tendency toward, 

 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 favor 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 numer- 

 ous, and come into play at different periods, so that it is quite pos- 

 sible for a germ-cell to have its future fate more than once changed. 

 The constitution of the mother, the nutrition of the ova, the constitu- 

 tion of the father, the state of the male element when fertilization 

 occurs, the embryonic nutrition, and even the larval environment in 

 some cases, — these and yet other factors have all to be considered. 



Some observations by Laulanie as to the embryonic organs are of 

 interest in this connection. He distinguishes both in birds and mam- 

 mals three stages in the individual development of the reproductive 

 organs. These he calls (1) Germiparity, (2) Hermaphroditism, (3) 



