J. W. H. Harrison 19 



There is yet one possible method of mitotic dislocation of the sex* 

 chromosomes. It must not be lost sight of that we are here discussing 

 a hybrid insect and, therefore, that the conditions in mitosis are not 

 normal. Suppose that the F chromosome, owing to the composite build 

 of the cells, is late in dividing, and, as a result, the split halves lag long 

 enough on the spindle to be enmeshed in the protoplasm of one daughter 

 cell alone ; on the contrary, imagine the X chromosome to divide 

 normally. Then we should obtain two cells, one in character XO and 

 the second XY'Y' (using Y' to designate the split halves of the original 

 F chromosome). Then, if once more the viability of the cells were not 

 alike, and the A^O cell were incapable of existing, we would have the 

 work of perfecting the organism thrown upon one of the first cleavage 

 cells alone and that the one of sex chromosome value XY'Y', In this, 

 through the inability of the split A'' to overcome the effects of the two 

 halves of the F, the cumulative potencies of the latter pair might result, 

 if segmentation proceeded, in the building up of a female. 



Even assuming that, exceptionally, in the gametogenesis of the 

 pomonaria-isahellae hybrids the sex chromosomes refuse to pair in the 

 reduction division, and thus the fertilisation of a zonaria female-producing 

 ovum yields a zygote including three sex chromosomes and is therefore 

 of composition XXY, mitotic dislocation of the same type can still 

 produce a female. It is quite conceivable that, in the pomonaria- 

 isahellae-zonaria zygote, owing to the presence of a multiplicity of 

 chromosomes in a cell intended for the movements of 56, they impede 

 one another in mitosis and produce a disjoining of the sex chromosomes 

 ending in the continued existence, as before, of cells of female type alone. 



A study of the above arguments will show that the progressive 

 degradation (or uplift) of the females through intersexes into males 

 in the primary hybrids referred to above may not inconceivably be due 

 to some incompatibility between certain chromosomes and the cells 

 containing them ; though this, with due regard to the perfect transition 

 chain, is not very likely. 



On the Preponderance of Males in the Poecilopsis pomonaria Male x 



Lycia hirtaria Female Broods when the L. hirtaria were of Scotch 



Origin. 



A further puzzling sex phenomenon was the huge excess of males in 



the hirtaria $ x pomonaria ^ broods when the hirtaria used were of 



Scotch origin, in contrast with the equality of the sex numbers when 



the hirtaria were English. 



2—2 



