I9IS-] THE HEREDITARY MATERIAL. 145 



hereditary factors, — one derived from each of its parents. Men- 

 del's law postulates further, that, in the ripening of the germ cells, 

 the members of each pair separate (Fig. i). Each mature germ 

 cell comes to contain but a single element (or factor) of each kind. 



Fig. I. Diagram to illustrate segregation of factors. The four pairs of 

 factors represented in the upper circle by AA, BB, CC, DD, undergo segrega- 

 tion so that each germ cell comes to contain one member of each pair. 



Now students of cytology had quite independently come to this 

 same conclusion in regard to the germ cells. They had found that 

 each cell contains a definite number of chromosomes, and that there 

 are two of each kind of chromosomes in every cell, — one from each 

 parent (Fig. 2, a). It had been found that at the ripening of the 

 germ cells the members of each pair of chromosomes conjugate 

 (Fig. 2, h), and then separate from each other (Fig. 2, c) , so that 

 each mature germ cell comes to contain but a single set of chromo- 

 somes (Fig. 2, d). Furthermore, students of experimental em- 

 bryology had obtained independent evidence pointing to the chromo- 

 somes as the bearers of the hereditary materials. 



We find, then, that cytologists had discovered a mechanism 

 in the cell that they had reason to think was the bearer of the 

 hereditary materials, and that the mechanism fulfills the essential 



