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ANNUAL REPORT SMITHSONIAN INSTITUTION, 1930 



of different kinds of pills. The pills with different potencies might 

 represent the ultimate hereditary factors, and the vials which held 

 them miglit represent the chromosomes. The comparison is far from 

 perfect, but at any rate the chromosomes are believed to contain each 

 a single row of unit factors arranged like beads on a string. Although 

 these factors are ultramicroscopic in size, their relative position in the 

 chromosome may be determined indirectly by a number of methods. 

 Throughout the vegetative parts of the plant, there are two of 

 each kind of chromosomes as shown diagrammatically at the right 



Figure 1. — Diagrum of reproductive cycle in the Jiruson weed. There are 12 kinds 

 of chromosomes which are rod-shaped bodies carrying the hereditai-y factors. In 

 cells of the vegetative (2n) part of the plant, each chromosome is present twice. 

 In the male (cT) and female (?) sex cells (In), found in the pollen grains and 

 ovules, each chromosome is present only once 



of the bracket. Since there are two chromosomes in each of the 12 

 sets, this part of the plant is called the 2n or diploid stage. At 

 every division of a cell in the 2n stage, each chromosome divides 

 longitudinally into two bodies like itself, until the divisions preced- 

 ing the formation of the sex cells. 



Just before the production of the egg cells within the ovary and 

 the pollen grains within the stamens, the chromosomes fail to 

 undergo the usual process of division and one entire chromosome 

 from each pair goes to each of the two daughter cells, thus reducing 

 the number of chromosomes by one-half. In this " reduced," hap- 



