38 _ PHYSICAL BASIS OF HEREDITY 



is the case is, however, not a simple matter. It could not 

 have been made by Mendel or in the earlier days after the 

 rediscovery of Mendelism (1900). An attempt to furnish 

 this demonstration is given in Chapter XX. Assuming 

 the demonstration to be satisfactory, we reach the highly 

 important conclusion that segregation is not something 

 peculiar to hybrids, but something most readily demon- 

 strated by means of hybrids, and that in all probability the 

 germ-plasm is at first made up of pairs of elements, but 

 at the ripening of the germ-cells these elements (genes) 

 separate, one member of each pair going to one daughter 

 cell, the other member to the other cell. The mechan- 

 ism by means of which such a process might take place 

 had been known for several years before its relation 

 to MendePs principles of segregation was realized. This 

 mechanism is to be found in the conjugation and reduc- 

 tion j)rocesses that take place in the maturation of egg- 

 and sperm-cell. An account of this process is given in 

 the next chapter. 



