460 Organisms Are Products of 



A problem to solve. Has it occurred to 

 you that it is strange that the species 

 number of chromosomes should stay the 

 same even though two cells unite in fer- 

 tilization before each new generation is 

 formed? Why does not the fertilized egg 

 have a double set of chromosomes and 

 thus have twice the species number? Evi- 

 dently something happens that keeps the 

 number of chromosomes and genes from 

 being doubled during fertilization. This 

 something occurs within the cells before 

 they become eggs and sperms. In each 

 one of these cells the number of chro- 

 mosomes is reduced. The full or diploid 

 number of chromosomes is reduced to 

 the half or haplo'id number in sperm and 

 egg cells during their production. 



How sperm and egg cells are formed. 

 You will remember that while tissue cells 

 become differentiated in the developing 

 animal the geiTn plasm remains un- 

 changed. Later this germ plasm divides 

 and forms many prijiiary sex cells. Since 

 all the divisions have been mitotic, each 

 primary sex cell has the number of chro- 

 mosomes and the kinds of genes char- 

 acteristic of the species. As the young 

 animal develops, connective tissue and 

 some other kinds of tissues surround and 

 support the primary sex cells, forming 

 the organs known as the spermary or the 

 ovary. 



After a while changes begin to take 

 place in some of the primary sex cells of 

 the spermary or ovary. These primary 

 sex cells change into sperms or eggs. 

 Then the animal is ready for reproduc- 

 tion. The process by which a primary 

 sex cell becomes a sperm or egg is called 

 7/mturatio7i. During maturation there is 

 always one important step, an unusual 



Heredity and Environment unit ix 



type of nuclear division, in which the 

 number of chromosomes is reduced by 

 one half. This is called reductioji division. 

 It is quite different from mitotic division. 

 Do you remember reading that all nuclei 

 divide by mitosis, with one exception? 

 You have just read of the exception. 



Let us see what happens when a pri- 

 mary sex cell undergoes reduction divi- 

 sion. Chromosomes in every cell are pres- 

 ent in pairs. The two members of each 

 pair now come close together. They may 

 even twist about one another. Soon the 

 members of each pair separate and move 

 to opposite ends of the cell. Each pair of 

 chromosomes acts in this way. Then the 

 cell body divides. Thus from a primary 

 sex cell are formed two daughter cells 

 each of which has the half (haploid) 

 number of chromosomes and the half 

 number of genes (Fig. 407). Note that 

 there has been no splitting of chromo- 

 somes as in mitosis. Furthermore, note 

 that this separating of chromosomes does 

 not occur in a haphazard way. Each 

 daughter cell normally receives one of 

 each original pair of chromosomes. It has, 

 therefore, only one set of genes in- 

 stead of rsvo. The daughter cell after a 

 few more changes becomes a gamete. In 

 this way the gamete has only half as 

 many chromosomes as the primars^ sex 

 cell or any other cell in that animal. In 

 males the daughter cells undergo a num- 

 ber of mitotic divisions,* producing a 

 large number of cells with the half num- 

 ber of chromosomes and a single instead 

 of a double set of genes. These cells then 

 change in shape and thus become sperms. 



* In some animals these mitotic divisions 

 occur in the primary sex cells before reduction 

 division takes place, instead of after it. The re- 

 sult is the same. 



