340 T. H. Morgan 
on the evidence that “half the spermatozoa are characterized by 
the presence of a special nuclear element which I shall call the 
“X-element’ while the other half fail to receive this element.” 
In the simplest case the X-element is a single chromosome. 
In other cases the X-element is the larger of a pair of chromo- 
somes—the smaller called the Y-element. In still other cases 
the X-element may be represented by two or more chromo- 
somes. Comparison of the somatic number of chromosomes in 
male and female shows that the X-element (whether a single 
chromosome or more than one) “‘is present as a single unit in the 
male while in the female it is doubled.” 
Wilson provisionally formulates the following theory. “ Males 
are produced from zygotes that contain but a single X-element; 
females from those that contain two such elements.” In ordinary 
sexual reproduction all the fertilized eggs should after maturation 
bear the male tendency because one X-element is left in the egg 
after reduction. If capable of parthenogenesis with the reduced 
or haploid number of chromosomes, such eggs should produce 
males, as appears to be actually the case in the bees and ants. 
If fertilized by a spermatozoon that lacks the X-element, the egg 
still produces a male for the same reason. If fertilized by a 
spermatozoon that contains the X-element, the egg produces a 
female because of the introduction not of a dominant ‘female 
tendency”’ but of a second X-element. 
Wilson points out that there may exist also other factors still 
unknown and calls attention to the formation of the asexual spores 
in mosses where there occurs an “apparent disjunction of the 
sexual tendencies” since these spores contain the reduced number 
of chromosomes. Nevertheless “we are led to suspect. : 
from the facts known in animals that the male-producing spores 
may be characterized by the absence of some element that is pres- 
ent in the female-producing ones.” ‘This difficulty is now consider- 
ably lightened by the discovery of Baltzer (see below) that in the 
sea urchin there may exist a pair of chromosomes of unequal sizes 
associated with sex. If such exist in the mosses their separation at 
reduction might lead to the formation of two kinds of spores, even 
if both have the same number of chromosomes. ‘The question, 
