212 GENETICS IN RELATION TO AGRICULTURE 
shown in the diagram, these eggs extrude a polar body, but the division 
is not reductional for the eggs all have four sex-chromosomes, the same 
number as the mother. These eggs hatch without fertilization into the 
winged migrant females. Of these there are two kinds, those which lay 
large eggs and those which lay small eggs, and, moreover, all which come 
from the same gall and, therefore, from the same stem mother lay the 
same kind of eggs. Accordingly the stem mothers are of two kinds with 
respect to their chromosome content as illustrated in the diagram. 
The female producing stem mothers are XxX and the male producing 
stem mothers are XxX2’, and the migrant females have the same chro- 
mosome content as the stem mother from which they were derived. 
The migrant female of the type XzXz produces large eggs which throw 
off a polar body, but do not undergo reduction. The resulting egg de- 
velops without fertilization into a minute sexual female. The other type 
of migrant females, however, lays small eggs in which, prior to extrusion 
of the polar body, the large X’s and the small x’s conjugate. One of each 
of these pairs then passes out into the polar body, so that two types of 
eggs are produced Xa and Xz’ and these develop without fertilization 
into the minute males. In the sexual females a true reduction division 
takes place so that her single egg is of the chromosome constitution Xz. 
The males on the other hand produce sperm cells half of which are Xx 
or Xz’ according to the type of male and half of which have none of the 
sex-chromosomes. Sperms of this latter type degenerate, so that. only 
female producing sperm remain. When these fertilize the sexual egg 
the resulting eggs are either XxXz or XxXv’, and give rise to the corre- 
sponding type of stem mother. This completes the complicated life 
cycle in this form, and illustrates again the close dependence of sex- 
determination on chromosome content. 
In plants only a few cases of sex-inheritance have been studied and 
these for the most part inadequately. Two of these, namely Bryonia 
and Lychnis, appear to display the XY type of sex-inheritance, but in a 
somewhat modified form. Thus Correns crossed Bryonia alba, which 
is moneecious, with Bryonia dioica, which is dicecious. The former 
species as a rule bears male and female blossoms on the same inflorescence, 
the female above and the male below, whereas the latter species con- 
stantly bears all male or all female blossoms on the same stem. Correns 
summarizes his results under four heads as follows: 
1. Female plants of Bryonia dioica pollinated by male plants of the 
same species give approximately equal numbers of male and female plants. 
2. Female plants of Bryonza dioica pollinated by Bryonia alba give 
only female offspring. 
3. Bryonia alba pollinated by male plants of Bryonia dioica gives 
approximately equal numbers of male and female plants. 
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