Reproduction in Multicellular Animals - 387 



FALLOPIAN TUBE 



URETER 



INTESTINE 



PUBIC 

 SYMPHYSIS 



URETHRA 



RECTUM ANUS 



Fig. 21-9. Genital tract of human female. 



ovary — in size and shape — resembles a shelled 

 almond (Fig. 21-9), although in older indi- 

 viduals the ovarian surface is roughened and 

 pocketed with numerous scars. These scars re- 

 sult from a repeated rupturing of the ovarian 

 wall during successive ovulations. 



The human ovaries usually produce only 

 one egg during each 28-day cycle of activity. 

 The egg cell arises from the germinal epi- 

 thelium of the ovary (Fig. 21-10). This epi- 

 thelium proliferates periodically, forming a 

 group of cells that is pushed inward from 

 the surface of the ovary and finally pinched 

 off as a separate mass (Fig. 21-10). This mass 

 represents an incipient Graafian follicle, 

 which soon becomes organized into a vesicle, 

 enclosing the maturing egg (Fig. 21-10). The 

 Graafian follicle continues to enlarge until 

 it protrudes slightly from the surface of the 

 ovary. Then, finally, when the egg is ripe, the 

 follicle ruptures, discharging an egg near 

 the mouth of the Fallopian tube. This proc- 

 ess of ovulation occurs periodically in the 



human female, usually at a fixed time in 

 relation to the menstrual cycle (p. 401). 



The ovary has also developed two impor- 

 tant endocrine functions, which are associ- 

 ated mainly with the Graafian follicles. As 

 each follicle ripens, it becomes filled with a 

 follicular fluid, which contains the "female 

 sex hormone," estrogen (p. 400). Estrogen has 



GERMINAL 

 EPITHELIUM 



D0WNGR0WTH 

 OF EPITHELIUM 



GRAAFIAN 

 FOLLICLES 



Fig. 21-10. Section of ovary. 



