42 GERM-CELL CYCLE IN ANIMALS 



of chromosomes and the amount of chromatin in 

 the germ cells. Typically, both male and female 

 germ cells divide twice during the process of matura- 



PRIMORDIAL / • • 



GERM-CELL ~ !•• 



MULTIPLICATION 

 PERIOD 

 SPERMATOGONIA^-- — VZ_Z/ \^-^^ 



^T^ f^»\ ("^^ /T7\_ 



/ GROWTH 

 > PERIOD 

 PRIMARY J I 1 ^ 1 



SPERMATOCYTE 1 / ^ 



SECONDARY ^ 



SPERMATOCYTES "W 



MATURATION 

 _ , PERIOD 



SPERMATIDS r.^ (••)(" 



SPERMATOZOA 



PRIMORDIAL / • • 



GERM-CELL ' \m %. 



MULTIPLICATION 

 • • ^ / PERIOD 



OOGONIA 



^^\ // X ¥ \v I 



''%/ "^ /^~~\ /' — N T"" X I 



{• •\ f% •\ (• •\ [• •] J 



\**y v*V \?V VTV'x 



GROWTH 



PRIMARY / . . \ r PERIOD 



OOCYTE I • # 



SECONDARY 

 OOCYTES 

 (OVARIAN EGG \ • • ) ^ V MATURATION 



AND POLAR BODY) V J /A / PERIOD 



MATURE EGG 



AND I , 



POLAR BODIES ' 



Fig. 9. — Diagrams illustrating (above) the stages of spermatogenesis 

 and (below) of oogenesis. The primordial germ cell is represented 

 as possessing four chromosomes. 



