HEREDITY OF SEX 271 
goes, where more than one individual develops from 
the same fertilized egg all are of the same sex seems 
to point conclusively in this direction ; and further 
valuable evidence has recently been adduced. Prof. 
E. B. Wilson has recently investigated the behaviour 
of the chromosomes in the somatic cells and in the 
germ-cells of a particular species of insect known as 
Protenor belfragi. The case afforded by this animal 
is remarkable inasmuch as the somatic cells in the 
male, and only in the male, contain an odd number 
of chromosomes. An irregularity is accordingly intro- 
duced into the process of fusion of the chromosomes 
in pairs, which, as already described, always precedes 
the formation of the germ-cells with their reduced 
number of chromosomes. In the case of the male 
Protenor all the chromosomes fuse in pairs except one, 
which is, of necessity, left over. This odd chromosome 
is described as the heterotropic chromosome. The 
female Protenor has one more chromosome in its 
somatic cells than the male, thus making up an even 
number; that is to say, in the female the pair to 
the odd chromosome of the male is present, so that 
there are two heterotropic chromosomes, or idio- 
chromosomes. These fuse and separate in the reducing’ 
division, which thus proceeds in the normal manner 
in this sex. In the male, on the other hand, when 
the reducing division occurs, the heterotropic chromo- 
some passes complete into one of the resulting cells. 
In the second gamete-producing division, every chro- 
mosome present having divided into two, the products 
