22 THE ORIGIN OF PLANT AND ANIMAL CHILDREN 
When the fertilized egg lodges in the uterus, it sends out 
little projections (villi) which fasten in the lining of the uterus 
and absorb from it their nourishment. In a few days the 
embryo grows so much that it needs a blood system to distrib¬ 
ute the supplies to all parts of its body. The villi on one side 
of it together with the 
lining of the uterus at 
the place of attachment 
develop into the placenta 
(called after-birth because 
it is expelled a little while 
after the birth of the 
child), an organ filled 
with blood vessels. There 
is a cord reaching from 
this placenta to the um¬ 
bilicus (navel) of the foe¬ 
tus. Through this cord 
run two arteries from 
the foetus to the pla¬ 
centa, and a vein return¬ 
ing. In the placenta the 
small blood vessels of the 
foetus entwine with those 
of the mother. The blood 
of one does not mingle 
with that of the other. 
There is always a thin 
membrane between them. Through this membrane the food 
and oxygen pass to the embryo’s blood and the wastes, car¬ 
bon dioxid and nitrogenous substances, pass into the mother’s 
blood for elimination. 
The human foetus is carried in the uterus nine months, the 
gestation period. This period is as short as a few weeks for 
some of the small mammals and nearly two years for the ele- 
Figure 17. —Section through a Uterus. 
The foetus is about two months old and is 
about one half the natural size. 
