Mender s Law 41 



The essential feature of Mi;ndi;i/s theor}- is his con- 

 ception of the purity of gametes, brought about by the 

 segregation of alternative characters. With Mendel 

 this was a purely theoretical scheme, but since his time 

 cytological investigation has discovered an actual 

 physical mechanism which exactly satisfies the require- 

 ments of Mendel's scheme. Every li\ang organism is 

 composed of cells, and these cells are endowed with 

 nuclei. Every nucleus contains a certain number of 

 darkly staining bodies known as chromosomes. The 

 number of chromosomes is always tlie same for a gi\'en 

 species. At the cell divisions which take place in con- 

 nection with the growth of the body, each chromosome 

 is very carefully divided in half, so that the nucleus of 

 each daughter-cell has exactly the same equipment of 

 chromosomes as the mother-nucleus. The exactness 

 of this division in itself suggests that the chromosomes 

 are the bearers of hereditary characters, since none of 

 the other cell constituents seems to be so accurateh' 

 divided at cell division. Even more significant is the 

 behavior of the chromosomes in connection with gamete 

 formation. At that time it becomes evident that the 

 chromosomes exist in pairs; thus there is always an 

 even number of chromosomes in every bcxh' cell of an 

 organism. The two components of each pair of chromo- 

 somes are always morphologically identical. When tlie 

 organism forms gametes, a cell dixision takes place which 

 is fundamentally different from the preceding cell divi- 

 sions. At this division no splitting of the individual 

 chromosomes takes place; instead, the chromosomes 

 Hne up in pairs and the nature of the division is such as 

 to draw apart the components of each ])air. This is 



