STUDIES IN THE EXPERIMENTAL ANALYSIS OF SEX. 579 
comprehensive to cover the fact, but it involves two large 
assumptions for which there is little or no evidence, and its 
very comprehensiveness prevents its affording any satisfactory 
explanation of the undoubted variations in sexual constitution 
which occur in Nature. 
The two assumptions, firstly, that selective fertilisation 
occurs, and secondly, that ’dominance is reversed in two sets 
of individuals in the same species, for some unknown reason, 
are not beyond the bounds of possibility, but they are 
unwarranted and cumbrous additions to Mendelian theory 
which we would gladly avoid, at any rate until all attempts 
at a simpler explanation have failed. 
The essence of a simpler interpretation was first hit upon 
by McClung in 1902 (3), althoug-h he did not express him- 
self in Mendelian language or definitely formulate a Mendelian 
theory of sex. McClung observed that in certain kinds of 
insect the males produced two forms of spermatozoa in equal 
numbers, one half the spermatozoa containing an accessory 
chromosome and the other half lacking it. This peculiar 
distribution of the accessory chromosome was found to be 
effected in the maturation division of the spermacytes, the 
accessory chromosome, instead of dividing, passing bodily 
over into half the secondary spermacytes. In the maturation 
of the ova no accessory chromosome was observed. McClung 
clearly pointed out that the behaviour of this chromosome in 
the male was in some way connected with the determination 
of sex. He considered that the spermatozoa containing the 
accessory chromosome on fertilising an egg gave rise to a 
male, while the normal spermatozoa gave rise to females. 
It is clear that, although McClung did not state it in this 
way, we have here the essence of a Mendelian theory of sex, 
according to which the male is a heterozygote of composi- 
tion, ^ ? , giving rise to ^ and ? gametes, while the female 
is a recessive ? ? , giving rise to pui-e ? gametes. This 
would further account for the production of the two sexes in 
equal proportions according to the laws of chance. Unfortu- 
nately this simple account of the matter has been proved 
VOL. 54, PART 4. — NEW SERIES. 41 
