160 
ZOOLOGY: C. E. McCLUNG 
POSSIBLE ACTION OF THE SEX-DETERMINING MECHANISM 
By C. E. McClung 
Zoological Laboratories, University of Pennsylvania 
Communicated by H. H. Donaldson. Read before the Academy, November 20, 1917 
Sex is the dual expression in organisms of a common series of characters 
which are unequally and reciprocally developed in the individuals of the two 
classes, the essential and primary difference between which is the presence of 
germ cells of two types — ova in the female, sperm in the male. Sex is not a 
necessity for reproduction or for biparentai inheritance: it is required in re- 
production of highly complex organisms. It exhibits many modifications of 
appearance and intensity in different groups and ranges from apparent cor- 
respondence of the two classes to wide extremes of dimorphism. While this 
gradation prevails for organisms in general, the individual is usually fixed in 
its status of differentiation. In exceptional cases the individual may possess 
the primary characters of both sexes or may alternately exhibit one or the 
other. The measure of difference in any case is cellular. The first observable 
cellular difference consists in a differentiation of conjugating cells into larger 
passive food-laden ova, and smaller, active sperms. Similarly the individual, 
ripe germ cell of either class appears definitely fixed in its character. The 
primary measure of difference between the two types of germ cells is now 
demonstrated to be nuclear, and, specifically, chromosomal. Sex is thus shown 
to be a cellular problem. 
Any conception of the sex determining mechanism must conform to the 
different conditions of range and intensity of sexuality manifested by organisms. 
As the conditions in one type of sexuality can not be substituted for another, 
so the character of the determinant in one case can not be implied directly 
in another. The details of the mechanism are variable in correspondence 
with the conditions it determines. 
In one type, in which the relation was first determined, the alternative 
character is definitely fixed and the body cells are individually and independ- 
ently male or female in type. The mechanism of determination here shows 
itself to be characterized by a difference of one chromosome more in the female 
than in the male. This is a particular chromosome, marked by such peculiari- 
ties in the male as to make it readily determinable. Since this element is the 
measure of the differences between the male and female types of cell, and 
therefore the measure of the difference between the male and female animal, 
a study of its differential behavior should be of value in arriving at conclusions 
regarding the nature of sex and the method of its determination. 
The two principal observations upon which an explanation of the operation 
of the sex determining mechanism must rest are (1) the duplex and alternative 
chromosome series, paralleling the double control and alternative inheritance 
of characters, and (2) the physical state of the active chromosomes. Sex, 
