THE CELL AS A FUNDAMENTAL UNIT OF LIFE 



31 



individual differences of the different members of the same species, but 

 the total chromosome number will be the same. There may be con- 

 siderable variation in the chromosome number, however, among mem- 

 bers of different species. For instance, a domestic chicken has 18 

 chromosomes in its body cells, a dog has 22, a bullfrog has 26, a fruit 

 fly has 8, and man has 48. 



The fertilized egg (zygote) which produces a person contains 48 

 chromosomes ; after this cell is duplicated each of the two cells formed 



Fig. 2.7. How the problem of chromosome number is solved. Although human body 

 cells typically have 48 chromosomes, this number is reduced to one-half (24) when the 

 reproductive cells are formed. When these unite and produce a new life, the full 48 

 chromosome number is restored. Thus, the number remains constant for the species 



from generation to generation. 



will contain 48 chromosomes, thanks to the exactness of mitosis. 

 Through continuing mitosis, this chromosome number is held constant 

 so that the cells of the brain, skin, stomach, liver, and other body organs 

 each will contain the same 48 chromosomes. When the time comes for 

 reproduction, however, an important problem arises. A new human 

 life is produced by a union of two cells (the sperm and the egg) from 

 two different persons. If both of these cells contained 48 chromosomes 



