SEX 357 



depends here upon the type of male gamete functioning, as is known 

 to be the case in so many animals. It has been suggested by Allen (1919) 

 that the separation of the sex-factors in dioecious seed plants may possibly 

 occur in the division which differentiates the two male nuclei in the 

 pollen tube, rather than at the divisions producing the microspores. 

 'That this interpretation cannot be applied to Mendelian factors in general 

 is evidenced by the fact that in maize hybrids the embryo, with very 

 rare exceptions, has been found to be like the endosperm with respect 

 to factors introduced by the pollen parent. So far as these factors are 

 concerned, therefore, the two male nuclei must be qualitatively similar. 



Strasburger's conclusion regarding monoecious mosses is confirmed 

 by the recent experiments of Collins (1919) on Funaria hygrometrica. In 

 this species the gametophytes arising from spores are bisexual (monoe- 

 cious), but if gametophytes are produced by regeneration from the 

 antheridia or perigonial leaves of a single "male flower," they all bear 

 antheridia only. Collins thinks it possible that dioecism may have 

 arisen as a result of vegetative multiplication following such a somatic 

 segregation in the tissue of the monoecious gametophyte. 



In animals also there is much evidence, aside from that afforded by 

 the chromosomes to be discussed below, in favor of the view that sex is 

 internally controlled. The following illustrative cases may be cited. 

 The egg of "the bee may develop either parthenogenetically or after 

 fertilization by a spermatozoon: in the former case a male (drone) results 

 and in the latter a female (queen or worker, depending on the nature of 

 the food). In Phylloxera (Morgan 1906, 1908, 1909, 1910) there are two 

 sizes of eggs produced by the females of the second parthenogenetic 

 generation: both may develop parthenogenetically after forming one 

 polar body, the larger ones into females and the smaller into males. 

 Fertilized eggs always develop into females. In Hydatina (Whitney 

 1914, 1916, 1917) the female-producing eggs form one polar body while 

 the male-producing eggs form two. Two kinds of eggs are also produced 

 in Dinophilus (Malsen 1906; Nachtsheim 1919), but in this form both 

 are regularly fertilized. In the nine-banded armadillo (Newman and 

 Patterson 1909, 1910) one fertilized egg commonly gives rise to four new 

 individuals, and the four are invariably all male or all female. Analogous 

 instances of polyembryony are also known in insects. Human twins, 

 if "identical" (produced by the same egg), are invariably of the same 

 sex; if "fraternal" (produced by different eggs) they may or may not be 

 of the same sex. It would therefore seem that sex in such cases as these 

 must be determined either in the egg before fertilization or at the moment 

 fertilization occurs. 1 



1 The determination of sex in the egg before fertilization, as in Phylloxera and 

 Dinophilus, is termed by Haecker "progamic" sex-differentiation; if determination 

 occurs at the moment of fertilization, as in the bee, it is "syngamic" sex-differentia- 

 tion; and if it occurs after fertilization, as may possibly be the case in some forms, it 

 is "epigamic" sex-differentiation. 



