136 THE MENDEL JOURNAL 



There are reasons for believing that the different 

 factors which determine the hereditary characters 

 of organisms are carried by certain nuclear bodies 

 called the chromosomes. In form they are some- 

 times rod-shaped and sometimes horseshoe-shaped, 

 and they make their appearance within the nucleus 

 just before cell division is about to occur. They stain 

 very darkly with certain dyes and their presence is 

 thus rendered more manifest. For the individuals 

 of a given species, the number of chromosomes 

 present in the nuclei of the cells is constant. But 

 the number of chromosomes in the body or somatic 

 cells is, however, twice that in the germ-cells or gametes 

 of any individual. Consequently, when at fertilisa- 

 tion two germ-cells, paternal and maternal, unite, the 

 resulting cell, out of which a new individual will 

 arise by cell division, contains the higher or somatic 

 number of chromosomes. Thence during all the 

 cell divisions which occur as the new individual 

 developes from this fertilised cell, this higher or 

 body number is retained in all the somatic cells 

 which result. But, at a certain stage, the individual 

 forms its germ-cells, and during the process the num- 

 ber of chromosomes becomes reduced to one half. 



Now there is clear evidence, derived from the 

 study of these chromosomes in certain insects, 

 phylloxerans, and aphids, to show that in some 

 way sex is directly determined, or its determination 

 is correlated with, the presence of an accessory 

 chromosome in certain of the paternal cells. 

 That is, if the egg-cells contain five chromosomes 



