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 consent as illustrated in the diagram. 

 The female producing stem mothers are XxXx and the male producing 

 stem mothers are XxXx', 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 XxXx 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 Xx and Xx' 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 Xx. 

 The males on the other hand produce sperm cells half of which are Xx 

 or Xx' 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 XxXx or XxXx', 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 -X'F type of sex-inheritance, but in a 

 somewhat modified form. Thus Correns crossed Bryonia alba, which 

 is monoecious, with Bryonia dioica, which is dioecious. 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 Bryonia 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. 



