GENERAL DISCUSSION. 55 



Here we know that such a combination of gametes must occur to 

 give the observed results, but we are not certain that we have a right 

 to attribute the sex characters to these particular chromosomes or in 

 fact to any chromosomes. It seems, however, a reasonable assump- 

 tion in accordance with the observed conditions. The scheme also 

 assumes either selective fertilization or, what amounts to the same 

 thing, infertility of gametic unions where like sex chromosomes are 

 present. It also assumes that the large female sex chromosome is 

 dominant in the presence of the male sex chromosome, and that the 

 male sex chromosome is dominant in the presence of the small female 

 sex chromosome. Or, it might rather be said that these are not really 

 assumptions, but inferences as to what must be true if the heterochro- 

 mosomes are sex chromosomes. This theory of sex determination 

 brings the facts observed in regard to the heterochromosomes under 

 Castle's modification of Mendel's Law of Heredity ('99). 



The question of dominance is a difficult one, especially in parthe- 

 nogenetic eggs and eggs which are distinctly male or female before 

 fertilization. It may be possible that the sex character of the egg after 

 maturation is always dominant in the fertilized egg, as appears to be 

 the case in these insects (see scheme). Conditions external to the 

 chromosomes may determine in certain cases, such as Dinophilus, 

 which sex character shall dominate in the growing oocyte, and matu- 

 ration occur accordingly. It is evident that this reasoning would lead 

 to the conclusion that sex is or may be determined in the egg before 

 fertilization, and that selective fertilization, or infertility of gametic 

 unions containing like sex characters, has to do, not with actual sex 

 determination, but with suitable distribution of the sex characters to 

 future generations. If both sex characters are present in parthenoge- 

 netic eggs, as appears to be the case in aphids and phylloxera, domi- 

 nance of one or the other must be determined by conditions external 

 to the chromosomes, for we have both sexes at different points in the 

 same line of descent without either reduction or fertilization. 



Wilson suggests as alternatives to the chromosome sex determinant 

 theory according to Mendel's Law, (1) that the heterochromosomes 

 may merely transmit sex characters, sex being determined by proto- 

 plasmic conditions external to the chromosomes; (2) That the hetero- 

 chromosomes may be sex-determining factors only by virtue of dif- 

 ference in activity or amount of chromatin, the female sex chromosome 

 in the male being less active. The first of these alternatives is an 

 attempt to cover such cases as Di?iophiliis, Hydatina, and Phylloxera 

 with large female and small male eggs. Here Morgan's ('06) sugges- 

 tion as to degenerate males seems much to the point. The male sex 



