THE CARRIERS OF THE HERITAGE 223 



6. MATURATION 



Certain preliminary changes of a preparatory na- 

 ture, termed maturation, regularly precede the union 

 of the nuclei of the two sex-cells in fertilization. 



These maturing changes result in reducing the outfit, 

 of chromosomes in each sex-cell to one-half the originalx 

 number, a process which is necessary in order to main-/ 

 tain the chromosomal count which is characteristic/ 

 for any particular species and which is known to exist 

 unbroken from generation to generation. If there were ; 

 no such reduction, then the fertilized egg, formed by 

 the union of egg and sperm nuclei, would contain double 

 the characteristic number of chromosomes, and during 

 the formation of a new individual, the number in all 

 the cells arising by mitosis from such a fertilized egg 

 would likewise be double. When the germ-cells of 

 such individuals unite in fertilization, the original num- 

 ber of chromosomes would be quadrupled, and so on in 

 geometric progression throughout subsequent genera- 

 tions. In 1883, too late for Darwin to learn of it, 

 van Beneden discovered the important fact that the 

 mature germ-cells, as expected, actually contain only 

 half the normal number of chromosomes. 



The mature egg- or sperm-cell, with half its normal 

 number of chromosomes, is termed a gamete (marry- 

 ing cell), while the fertilized egg which is formed by 

 the union of two gametes (mature egg- and sperm- 

 cell), and which consequently has the characteristic 

 number of chromosomes, is called a zygote (yoked cell). 



A diagrammatic representation of the process of 



