20 THE ORIGIN OF PLANT AND ANIMAL CHILDREN 
situated in the abdominal or in the pelvic cavity. In 
human beings the eggs mature (become ripe) about once a 
month, usually one at each period. 
On maturity the egg breaks through the membrane cover¬ 
ing the ovary and is received into the open end of the uterine 
Figure 16. — Section through the Female Pelvis. 
(Fallopian) tube. The uterine tubes, one on each side of the 
pelvis, are lined with cilia (small threads of protoplasm pro¬ 
jecting from the cells), which by their wavelike motion 
sweep the eggs to the uterus (womb), a sack with muscular 
walls lying in the middle of the pelvis. A small opening 
leads through the neck ( cervix ) of the uterus to the vagina. 
The semen from the male is received at sexual intercourse in 
