THE GENETIC SYSTEM 53 



(As will be shown later, the number of diverse types of 

 germ cells producible is greatly increased beyond these fig- 

 ures, by the fact that the two chromosomes of a pair may 

 during conjugation exchange parts in various proportions. 

 See Chapter VI) . 



27. Thus any single individual produces many diverse 

 types of germ cells, containing diverse combinations of its 

 chromosomes. This is true both for the sperms and the ova. 



28. In fertilization, sperms and ova having the various 

 diverse combinations of chromosomes unite at random. 

 Thus the many fertilized eggs, and the offspring developed 

 from them, bear different combinations of chromosomes. In 

 consequence, as will be shown later, they may develop dif- 

 ferently, giving many diverse types of individuals. The 

 numbers of different types thus producible by a given pair 

 of parents is dealt with in later chapters. 



Certain Terms Defined: — In connection with the above 

 account of the history of the materials of heredity, it will 

 be well to observe the meaning of certain common terms, 

 whose use is very convenient; they will be employed in later 

 pages: 



Gametes: The mature germ cells (ova and sperm) are 

 known as gametes. In the gametes the chromosomes are 

 single instead of in pairs: that is, the gametes contain but 

 n chromosomes, in place of in. Cells having thus but one 

 set of chromosomes are said to be haploid, while the usual 

 cells, with chromosomes in pairs, are diploid. 



The entire process of producing gametes is known as 

 gametogenests. The later processes, by which the four ma- 

 ture germ cells are produced, are spoken of as maturation. 

 The chromosomal changes during maturation, resulting in 

 reduction in the number of chromosomes from m to n, 

 are known as meiosis. 



Zygote: The fertilized egg, formed by the union of two 

 gametes, Is the zygote. This term is often applied to the 



