312 ELEMENTS OF BIOLOGY 



kept constant throughout the numerous cell generations as the indi- 

 vidual develops and grows from the fertilized egg to the adult. The 

 manner of transmitting chromatin from parent to offspring is the 

 critical process in the passing of inheritable characters from one 

 generation to the next; our discussion of this process will complete 

 the account of the essentials of the history of the chromosomes 

 within the species. ' 



The Chromosomes of the Zygote. Fertilization of the egg M 

 by the spermatozoon combines cells from two individuals. If both ■ 

 gametes were to contain the S number of chromosomes, the number 

 in the new individual would be twice S, and with each succeeding 

 generation doubled again. Shortly the number of chromosomes in 

 the species would approach infinity, unless some sort of process 

 reduced the chromosome number in the gametes. This reduction 

 actually occurs in the ripening and maturing processes through 

 which both sperm and ova pass before they are capable of fertili- 

 zation. Consequently, when two gametes unite, each has but one- 

 half the S number of the species ( ~ ); the union of the egg and 



sperm nuclei thus restores the normal number of the soma. The 

 process through which the chromosomes pass during the maturing 

 of the germ cells is complex, but in order to understand the basic 

 principles of inheritance it is necessary to comprehend it in some 

 detail. The maturation of the spermatozoa will be described first. 

 Spermatogenesis (Fig. 201). The germ cells are set aside very 

 early in the life history of the organism; in some animals they may 

 be identified during the very first stages of cleavage. In the male 

 mammal the primordial germ cells come to be placed in the walls 

 of the seminiferous tubules; in lower forms they are located in 

 other types of testicular tubes or follicles. Here by ordinary mitotic 

 cell divisions they increase in number. During this period of multi- 

 plication they are known as spermatogonia. Presently, however, 

 they cease to divide and enter a period of extensive nuclear change 



