THE DETERMINATION OF SEX 339 



the life of an individual, instead of annually or more frequently 

 as in manj^ of the flowering plants and some of the lower animals. 

 The degree of differentiation of the primitive ind(4(M-minate 

 structures in the higher animals also is extreme. Although the 

 sexes of the birds retain a fairly ])rimitive condition of the repro- 

 ductive organs which offers no structural obstacle to reversal, the 

 highly differentiated organs of mammals can hardly be expected 

 to submit to such an extreme change. It is probable that if the 

 mammalian embryo could be subjected to the proper stimulus 

 while its sexual organs were still in the indeterminate stage its 

 sex might be controlled. The freemartin most nearly attains this 

 condition, but since male hormones appear to be the determining 

 factor it is evident that sexual differentiation must precede the 

 formation of these substances; the female must be definitely 

 female before it is subjected to the action of the hormones of its 

 male twin. 



The entire subject is one of little practical importance. One 

 sex is sometimes more useful than the other among both plants 

 and animals and man has alwaj'S shown a desire to predict or 

 control the sex of his own offspring, but there is so Httle possibility 

 of simple and effective methods of controlling sex that at present 

 it has only scientific value. In this it is of primary importance 

 as a demonstration of chromosomal functions. 



Summary. Sex is not essential to reproduction as a biological 

 phenomenon, although in most of the higher organisms reproduc- 

 tion is accompanied by sex phenomena. Both types of germ cells 

 and the sexes that produce them are equivalent, and their differ- 

 entiation must therefore be explained by the specialization of the 

 reproductive process. The most important result of sexual repro- 

 duction is the recoml^ination of the unit characters making up 

 the species. These conclusions are borne out by a large number 

 of unusual cases, both natural and artificial, including hermaph- 

 roditism, gynandromorphism, the effects of endocrine imbal- 

 ances and sex reversal. All available evidence combines to show 

 that sex is in many cases determined by the chromosomes but 

 that these require definite conditions for their normal expression. 

 Whether these conditions are also determined by the body or 

 come from without, their modification may be expected to influence 

 the sex of the individual, but determinate sexual structures may 

 Umit the extent of modification. 



