12 



HUMAN EMBEYOLOGY. 



The germ cells reach their full development in special sex glands, the ova in 

 the ovaries of the female and the spermatozoa in the testes of the male. 



After the descendants of the primitive germ cells have increased, by ordinary 



Primitive germ cell 



Young oocytes ( U 



Oocyte II and first 

 polar body 



Mature ovum and 



Period of Division 

 during which 

 numerous de- 

 scendants are 

 formed from the 

 primitive germ 

 cell. 



Period of Growth. 

 of oocytes I. 



Period of Matura- 

 tion during which 

 four descendants 

 are formed from 

 eachoocyte I, i.e. 

 the mature ovum 

 and three polar 

 bodies (the 

 number of polar 

 bodies is i r - 

 regula,r). 



FIG. 9. SCHEMA OF THE DEVELOPMENTAL HISTORY OF THE MATURE OVUM. 



Primitive genii cell 



Young 

 spermatocytes 



Spermatocyte II 



Spermatids 



Spennatozoa 



Period of Division dur- 

 ing which many de- 

 scendants are formed 

 from each primitive 

 germ cell. 



Period of Growth 

 spermatocytes I. 



Of 



Period of Maturation 

 during which four 

 spermatids are pro- 

 duced from each sper- 

 matocyte I. 



Period of Transforma- 

 tion during which 

 each spermatid be- 

 comes a spermatozoon. 



FIG. 10. SCHEMA OF THE DEVELOPMENTAL HISTORY OF SPERMATOZOA. 



cell division, to a number which is probably fixed and unchangeable, but which is 

 not definitely known, they begin to increase in size, that is, they enter upon a period 

 of growth, and at this time the female germ cells are called oocytes of the first order, 

 oocytes I, and the male germ cells are called spermatocytes of the first order, sperma- 

 tocytes I. Both the oocytes I and the spermatocytes I possess all the essential 



