THE GENETIC SYSTEM 47 



same set of chromosomes and genes as have the body cells. 

 (There are exceptions to this in certain organisms.) That 

 is, the primordial germ cell has n pairs of chromosomes, 

 making in (or 2n-i) chromosomes. Of these, n are ma- 

 ternal in origin, while n (or n-i) are paternal in origin. 



5. The primordial germ cell divides many times, to pro- 

 duce many immature germ cells. These still contain the 

 full two sets, n pairs, of chromosomes (D, E, figure 17). 



The immature germ cells in the male are the sperma- 

 togonia, while those in the female are oogonia. 



6. There is usually a period of youth of the organism, 

 in which the immature germ cells remain at rest, not divid- 

 ing actively. 



7. As the individual becomes mature, there is a renewed 

 multiplication of the immature germ cells, the chromosomes 

 remaining as before. 



Next follows a series of processes by which the final germ 

 cells are produced : in their totality they are known as matura- 

 tion. They include important changes in the chromosomes; 

 these changes taken together are known as meiosis. Matura- 

 tion and meiosis include the following: 



8. Growth Stage: Some of the immature germ cells now 

 grow, becoming much larger (figure 17, F). After becom- 

 ing large, the spermatogonia are called primary sperma- 

 tocytes, while the oogonia are called primary oocytes. 



9. Each primary spermatocyte and oocyte is later to 

 divide twice, producing four mature germ cells. 



While the growth and division are occurring, important 

 processes occur in the chromosomes, as follows (see fig- 

 ure 18) : 



10. The chromosomes pair if they have not already 

 done so; that is, the corresponding paternal and maternal 

 chromosomes place themselves side by side close together. 



11. While pairing, the chromosomes are in the form of 

 very long slender threads, which have thickenings at inter- 



