e there
would be expected to cause this peculiar non-disjunction of the X
chromosomes.

That this was the explanation was shown when the material was examined. The
females that gave these results were found by Bridges to have two X's and a
Y chromosome.

The normal chromosome group of the female is shown in figure 52 and the
chromosome group of one of the exceptional females is shown in figure 69.
In a female of this kind there are three sex chromosomes X X Y which are
homologous in the sense that in normal individuals the two present are
mates and separate at the reduction division. If in the X X Y individual X
and X conjugate and separate at reduction and the unmated Y is free to move
to either pole of the spindle, two kinds of mature eggs will result, viz.,
X and XY. If, on the other hand, X and Y conjugate and separate at
reduction and the remaining X is free to go to either pole, four kinds of
eggs will result--XY--X--XX--Y. As a total result four kinds of eggs are
expected: viz. many XY and X eggs and a few XX and Y eggs.

[Illustration: FIG. 69. Figure of the chromosome group of an XXY female,
that gives non-disjunction.]

These four kinds of eggs may be fertilized either by female-producing
sperms or male-producing sperms, as indicated in the diagram (fig. 70).

[Illustration: FIG. 70. Scheme showing the results of fertilizing white
bearing eggs (4 kinds) resulting from non-disjunction. The upper half of
the diagram gives the results when these eggs are fertilized by normal red
bearing, female producing sperm, the lower half by normal, male producing
sperm.]

If such an XXY female carried white bearing Xs (open X in the figures), and
the male carried a red bearing X (black X in the figures) it will be seen
that there should result an exceptional class of sons that are red, and an
exceptional class of daughters that are white. Tests of these exceptions
show that they behave subsequently in heredity as their composition
requires. Other tests may also be made of the other classes of offspring.
Bridges has shown that they fulfill all the requirements predicted. Thus a
result that seemed in contradiction with the chromosome hypothesis has
turned out to give a brilliant confirmation of that theory both genetically
and cytologically.

HOW MANY GENETIC FACTORS ARE THERE IN THE GERM-PLASM OF A SINGLE INDIVIDUAL

In passing I invite your attention to a speculation based on our maps of
the chromosomes--a speculati