ZOOLOGY: C. E. McCLUNG 
161 
being a case of strict alternative heredity, would necessarily require a method 
of control which should unfailingly operate to produce one or the other condi- 
tion, and, since the numbers of males and females are approximately equal, it 
should also conform to this numerical requirement. The presence and be- 
havior of the accessory chromosome supplies the theoretical demands of such 
a control. It is exactly alternative in its apportionment to the male cells, 
producing two equal and differentiated classes; and, aside from their unequal 
speed of approach to the egg, and selective attraction by it, should go into 
exactly half the ova. This behavior has been determined accurately and is 
no longer questioned. 
The alternative mechanism is evident in the behavior of the accessory chro- 
mosome. It has not been clear why there should be a double condition of the 
sex chromosome in the female — in this respect conforming to the ordinary 
conditions — and a single representation in the male. The condition obtains 
however and in this fortunate circumstance we find the most promising ap- 
proach to an understanding of the essential character of sex and the method 
of its determination. Of great significance here is the fact that, in the mechan- 
ism of the germ cells, there exists nothing of functional value in one sex that 
is not contained in the other. For the differential element of the problem 
we have therefore to look, not for something that is in itself male or female, 
but for some factor which, in operation under one set of conditions, will so 
control a series of characters as to give it the aspect which represents maleness; 
under another set of conditions the alternative state of femaleness. How 
then does this differentiator function? 
The simplest conception of its action, perhaps, would be to ascribe to it 
some specific power which, exerted to a certain degree, might eventuate in 
the aspect of maleness, while in double that effect the series of characters 
would be female. In essence this is the quantitative theory. It does not 
however conform to any of our ideas of the alternative, or allelomorphic, action 
of the chromosomes. What our experience with regard to chromosome action 
dictates so far is this : One control for a character is sufficient for its elabora- 
tion; two controls do not exhibit the sum of their action but find themselves in 
opposition and usually one or the other prevails (probably for the single cell 
one always does) ; additional controls are apparently without effect. 
Genetic experiments also show that in cases of sex-linked inheritance a 
single control in the male has the same effect as a double identical control in 
the female, and that, if the factors in the female are opposed to each other, 
such a single control as is fully effective in the male is then only half as potent. 
The evidence accordingly relates to conditions of cell constitution and 
explanations must be in terms of cell organization and known functions. In 
this respect a comparison of the sex chromosomes of the male and female at the 
critical time when their germ cells are being prepared for union is most in- 
structive. While in the female there is apparent no unusual activities of the 
sex element, as compared with the other chromosomes, in the male there are 
