cytology: the study of the cell 



137 



or region in the cell as observed microscopically. This has been accomplished 

 by cytological investigation. When Mendel's paper was discovered in 1900 

 and the laws of the distribution of hereditary determiners were thus brought 

 to light, the cytologists found that they had already seen with the micro- 



fa) 



(d) 



Fig. 1. Diagrams to show the ordinary process of cell division (mitosis). — a. Four 

 chromosomes are shown in the spherical nucleus, each chromosome containing a 

 series of genes. Chromosomes drawn in black are descended from one parent, white 

 chromosomes are descended from the other parent. — b. Each chromosome splits 

 and becomes double. — c. The nuclear wall vanishes, a spindle-shaped area develops, 

 the split chromosomes become aligned across the middle of the spindle. — d. Forces, 

 the nature of which are not understood, pull the halves of split chromosomes apart. — 

 e. The daughter chromosomes are gathered into new nuclei and the cell is cut in 

 two. Each daughter cell is exactly like the parent cell in its chromosome make-up. 

 Each nucleus has two complete sets of chromosomes and genes, one descended from 

 one parent, the other from the other parent. 



scope structures in the cell whose behavior exactly corresponded to the be- 

 havior of the genes (fig. 1). It had been found that certain bodies which had 

 been termed chromosomes were indeed divided equally whenever a cell di- 

 vided, each chromosome becoming split longitudinally, one-half going to each 

 daughter cell. These and no other bodies in the cell were distributed with ex- 

 actitude to the daughter cells. It was found that the number of chromosomes 

 in a cell corresponded exactly to the number of blocks of genes existent in that 

 cell, that larger blocks of genes corresponded to larger chromosomes, and 



