THE CELL AS A FUNDAMENTAL UNIT OF LIFE 33 



then the child which was formed would have double this number (96) 

 in each of the body cells. The following generation would have double 

 this number and so on, until an impossible number of chromosomes 

 would be obtained. Hence, it is readily apparent that something must 

 intervene to hold the chromosome number constant from generation 

 to generation. This is done by a reduction of the chromosome number 

 to one half in the reproductive cells. The human sperm and the human 

 egg each contain only 24 chromosomes instead of the 48. A special 

 type of cell division, known as meiosis, takes place shortly before the 

 sperms and eggs are formed and accomplishes this reduction. Meiosis 

 is true cell division rather than cell duplication, because both the cy- 

 toplasm and the chromosomes are divided into two equal parts in this 

 process. The chromosomes line up in pairs at the metaphase and 24 

 of them move to each pole of the spindle before the cell divides. In 

 ordinary mitosis, on the other hand, you will recall that the chromosomes 

 line up in a row in the center of the spindle, are split to form 96, and 

 48 move to each end of the spindle. This is the basic difference be- 

 tween the two — in mitosis there is chromosome duplication followed by 

 cell division ; in meiosis there is separation of pairs of the chromosomes 

 without duplication, followed by cell division. Thus the chromosome 

 number remains constant from generation to generation with each 

 parent contributing one half of the chromosomes which go to form the 

 gene complex of a new life. In a later chapter (Chapter 31) we shall 

 learn more about the details of meiosis in the actual formation of the 

 sperms and the eggs. 



REVIEW QUESTIONS 



1. Tell how and by whom cells were discovered and named. 



2. Why do cells show a varicolored image when they are treated with several 

 different kinds of stains ? 



3. List the parts of a typical animal cell and the functions of each that were 

 given in this chapter. 



4. Distinguish between anabolism and catabolism. 



5. What is the relation between growth and cell duplication? 



6. Explain how the duplication and separation of genes are facilitated. 



7. Describe the events which take place in each phase of mitosis. 



8. Why are the chromosomes of different shapes during the anaphase? 



9. How do identical twins prove the duplication of genes in mitosis? 



10. Why are identical twins more alike than fraternal twins? 



11. What is the relation of research on mitosis to cancer control? 



12. How is the species number kept constant in a species from generation to 

 generation ? 



