The Sex Chromosomes. 265 
sponding difference between the nuclear constitution of the sexes 
in this respect.“ This is suggested by many facts, including, 
l. the tendencey of the X-chromosome in various species to break 
up into two or more separate components; 2. the conditions in 
Ascaris megalocephala, where the X-chromosome may be either 
separate or fused with one of the ordinary chromosomes; 3. the 
regular association between the X-chromosome and one of the 
ordinary bivalents, discovered by Sinety (1901) and Me Clung 
(1905) in certain Orthoptera. Under this view, the "small idio- 
chromosome‘“ is the more or less reduced free Y-member of the 
original pair, while the X-chromosome, its synaptic mate, not 
only contains the ”X-chromatin‘“ but may also contain at least 
certain constituents of the Y-chromatin. 
I have pointed out that this may give a basis for an 
understanding of sex-limited heredity. It has now become clear 
that in heredity of this type most of the facts become intelligible 
under the assumption that the sex-limited character is determined 
by some factor that is linked with, or contained in, the sex- 
chromosomes — in the X-chromosome when the male is digametie 
(Wilson, Morgan), in the synaptic mate of X when the female 
is digametic (Spillman). In the case of color-blindness, for 
example, all the facts seem to follow under this assumption if 
the male be digametic (as Guyer’s observations show to be the 
case in man). For, in fertilization this character will pass with 
the affected X-chromosome from the male into the female, and 
from the female into half her oftspring of both sexes (Diagram, 
Fig. 5). Color-blindnes, being a recessive character, should there- 
fore appear in neither daughters not granddaughters, but in half 
the grandsons, as seems to be actually the case. The same 
interpretation will apply equally to the heredity of white eye-color 
in Drosophila, as observed by Morgan. My more specific 
suggestion was that if such sex-limited characters be ascribed to 
specifie Y-constituents that have remained in association with the 
X-chromatin, a simple physical basis is provided for such a linking 
of these characters with the sexual ones. 
As to the operation of the quantitative factor represented 
by the X-chromosomes, nothing is really known. 
It is hardly to be doubted that the differences between the 
sexes in respect to the X-element must involve some corresponding 
