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 

 • • \ f • • ^ r PERIOD 



SPFRMATOGONIA^--"' \ • • / V • • 



• v <!_V V^V Uv 



GROWTH 

 PERIOD 

 PRIMARY / t 1 



SPERMATOCYTE ' * 



SECONDARY . , , ^, 



SPERMATOCYTES 



MATURATION 

 _ PERIOD 



SPERMATIDS (7>\ Cr>\ (i 



SPERMATOZOA ^ 



PRIMORDIAL , 



GERM-CELL \% * , 



MULTIPLICATION 

 • •> f * • '^ ^ PERIOD 



OOGONIA ^' 



N 



N 



N 



s 



'••\ ' ^ *\ [* *\ { * * 



f J V • •/ \« •/ \ • •> 



GROWTH 

 Pf'l^'ARY / \ ^ pERioo 



OOCYTE I • • 



SECONDARY 



OOCYTES 



(OVARIAN EGG "" ~ ~ ~\ * * } ^ V MATURATION 



AND POLARBODY) \ 7 /\ > PERIOD 



MATURE EGG /^ ^ \^ d. \_^ 



AND /^ , , ] P) rt ft 



POLAR BODIES \ 7 ^^ V_y ^-^ 



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

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

 as possessing four chromosomes. 



