108 



METAZOA IN GENERAL 



chromosomes, which were brought together in pairs in synapsis, are 

 separated again in one of these divisions, in which case, instead of each 

 chromosome dividing, whole chromosomes pass to the poles of the spindle. 

 Thus the number of chromosomes becomes reduced to half the number 

 contained in the primordial germ cell. This peculiar type of cell division 



Multiplica+ion 

 period 



Primordial germ cell 



and SO on for a very /arqe number of cell divis/ons 



QTaXJ any one spermatogonium 



,. -Synapsis 



Primary spermatocyte 



Me/ otic division 



^ I ] Secondary 

 — J Spermatocyte 



/j \*^ [ / \v\ Spermatids 



Sperm cells 



Fig. 41. — Diagram illustrating spermatogenesis, the haploid number of chromosomes being 



four and the diploid eight. 



is known as a reduction division, or meiosis. The reduced number of 

 chromosomes is known as the haploid number, while the larger number, 

 found in all somatic cells and in all immature germ cells, is called the 

 diploid number. Sometimes the reduction occurs in the first of these 

 maturation divisions and sometimes in the second. 



The spermatids undergo a process of modification or ripening which 

 involves a change in form and also the loss of a considerable amount of 



