AN INTRODUCTION TO THE METAZOA 



103 



tion of the primordial germ ceils or spermatogonia, (2) the 

 growth of these cells, and (3) their ripening or maturation. 

 These stages occur in all Metazoa, from the lowest to man. No 

 one knows how many cells are produced during the period of 

 multiplication. The last generation of spermatogonia gives 



PRIMORDIAL 

 GERM-CELL 



BPERMATOGONIA^.-' 



PRIMARY 

 SPERMATOCYTE 



SECONDARY 

 SPERMATOCYTES 



SPERMATIDS 



SPERMATOZOA 



MULTIPLICATION 

 PERIOD 



GROWTH 

 PERIOD 



MATURATION 

 PERIOD 



FIG. 49. Diagram illustrating the stages of spermatogenesis. The primor- 

 dial germ cell is represented as possessing four chromosomes. 



rise by division to the primary spermatocytes. The latter 

 increase greatly in size during the long growth period, and in 

 each of them the chromosomes unite or conjugate to form 

 double or bivalent chromosomes. Each primary spermatocyte 

 gives rise by division to two secondary spermatocytes. During 

 this division the chromosomes, which united to form the bivalent 

 chromosomes, separate, one single or univalent chromosome 

 going to each secondary spermatocyte. This is the only known 

 case in cell division where entire chromosomes are separated 

 from one another, except the corresponding stage in oogenesis. 

 It is known as a reduction division because it results in a re- 

 duction in the number of chromosomes to one half in the 

 daughter cells. The secondary spermatocytes immediately 



