40 



Inside the Living Cell 



chromosomes line up close to each other and exchange parts with 

 each other. A new chromosome is then formed which contains parts 

 from each of the original pair of chromosomes. This remarkable 

 process, which is known as crossing over, ensures that the single 

 chromosome of the germ cell contains parts from both parents, but 

 as the exchange can take place in many different ways, each germ 

 cell contains a different selection of the genes which the individual 

 carries and which it has received from its two parents. 



When fertilization occurs, the male and female germ cells unite 



Sperm 



Egg cell 



Fertilized 

 egg cell 



FIG. 6. Fertilization of the egg by the 

 sperm restores two chromosomes of 

 each kind to each cell (only one 

 chromosome in each germ cell is 

 shown) 



and the normal number of chromosomes of each kind — two — in the 

 cell is then restored. (Fig. 6.) A new individual is formed by many 

 divisions of the fertilized cell. In every division each chromosome is 

 replicated, so that every cell of the new organism contains two of 

 each kind of chromosome, which have been derived from the two 

 parents. This continues to be the case until the germ cells of the new 

 individual are formed, when the reduction process occurs again, 

 which again mixes in a single chromosome the genes derived from 

 both parents. 



This device, which permits two parents to contribute to the 

 heredity of a single individual, seems to have a crucial one for the 



