THE GENETIC SYSTEM 4 1 



while the two members of the same pair are of the same 

 size and form (figure 3). 



The paired condition of the chromosomes turns out to 

 be of great physiological importance. 



B. The Chromosomes in the Cell Divisions 

 that Produce the Body 



1. The egg divides into 2 cells, these into 4, and so on, 

 to 8, 16, and the like, till a great number of cells are formed, 

 making up the body of the adult organism. 



2. At each cell division, each of the 2n chromosomes 

 divides into 2, by splitting lengthwise (mitosis; see fig- 

 ure 2). 



3. One-half of each chromosome passes into each of 

 the two cells formed by division. 



4. There, each half grows to form a complete chromo- 

 some. Thus the chromosomes reproduce by fission, like 

 Protozoa. 



5. Consequently each cell formed contains the full set 

 of 2« (or 2«-i) chromosomes, half of them maternal, half 

 paternal. 



6. Ultimately therefore every cell of the body contains 

 the set of 2« (or in-i) chromosomes, half maternal, half 

 paternal in origin. (There are some exceptions to this state- 

 ment.) 



C. The Chromosomes Between the Cell Divisions, 

 When the Cells Are Not Dividing 



1. At the time of cell division the chromosomes are in 

 the condensed condition (shown in figures i, 3, 6, 7). 



2. After completion of cell division, each chromosome 

 absorbs fluid from the cytoplasm, grows, enlarges, and be- 

 comes a small vesicle. 



