CHROMOSOMES AND SEX 397 



indicates that all of the pollen grains carried an X, instead of half of them 

 X and half of them Y. There was also some evidence that a staminate 

 plant bearing carpels produced some eggs with X and some with Y. 

 In Melandrium album an exceptional bisexual individual was shown to 

 have XF in both micro- and megasporocytes. The same is true of a 

 bisexual Populus tremuloides.^'^ Thus under certain conditions it is 

 possible for reproductive elements of one sex to develop with the chromo- 

 some complement ordinarily characterizing the opposite sex. 



Such cases of sex modification in plants and animals^^ serve to empha- 

 size the fact that it is not simply the sex-chromosomes but a complicated 

 system of which the chromosomes are a part that is responsible for the 

 development of the sexes. The development of sex, like that of any other 

 character, depends upon the constitution of the protoplasm concerned 

 as well as upon the conditions to which this protoplasm is compelled to 

 react. Although the protoplasmic constitution is directly inherited, its 

 reactions in the course of ontogeny can be influenced within certain limits 

 through external agencies. Hence the determination of sex, like that 

 of other heritable characters, involves the interaction of two sets of fac- 

 tors: (1) genetic factors (inherited protoplasmic constitution; intrinsic 

 factors; genes); and (2) non-genetic factors (extrinsic factors; environ- 

 mental agencies, external and internal). 



The state of balance between those genetic factors whose influence 

 is toward maleness and those tending toward femaleness varies greatly 

 in different organisms. In bisexual forms it is so delicate that the 

 two sexes develop in response to different conditions prevailing in different 

 regions of the same individual, just as other distinct characters develop 

 near one another. In unisexual forms, especially in those with well- 

 differentiated sex-chromosomes, two unlike complexes of genetic factors 

 are established through meiosis and syngamy. The balance of male- 

 producing and female-producing factors in these two complexes is such 

 that, under average conditions, one of them predisposes development 

 toward maleness and the other toward femaleness. In some cases, 

 notably the bryophytes, this predisposition is very strong and has not 

 been overcome by environmental alterations so far tried; whereas, in 

 other cases, such as the angiosperms mentioned above, the natural 

 predisposition can be overcome more or less easily. In no case can the 

 action of either genetic or non-genetic factors be denied. It now seems 



^- McPhee (1925) on Cannabis, Belaf (1925) on Melandrium, Erlanson and 

 Hermann (1928) on Populus. 



*^ Among the many accounts of sex modification and its physiological significance 

 in animals may be cited those of Riddle (1912 et seq.; see 1927), Crew (1923, 1926, 

 1927abc), Banta (1916 ei seq.), Baltzer (1914), F. R. Lillie (1917), Minoura (1921), 

 Goldschmidt (1911 et seq.; see 1923a, 1927afe, 19286, 19296, 1931), Witschi (1922 et 

 seq.; see 1929 and 1932), Seller (1920), and Brambell (19305). 



