16 THE ORIGIN OF PLANT AND ANIMAL CHILDREN 
small, the oxygen of the atmosphere or in the water can pene¬ 
trate to all parts of it and supply its needs; but in the larger 
embryo there must be a special organ for breathing, and a cir¬ 
culating blood to distribute the oxygen and food to all parts 
of the body. While the bird embryo is still very small, it 
One set of blood vessels spreads over the yolk to absorb food and bring it 
into the embryo. Another set spreads out against the shell to take in oxygen 
and give out carbon dioxid. 
develops a thin membrane filled with blood vessels, which 
soon spreads over the inner surface of the shell. Through 
the porous shell the gases pass, oxygen coming into the em¬ 
bryo’s blood and carbon dioxid, the waste gas, going out. 
In this way the membrane acts as a gill or lung. 
1. How do very small embryos get oxygen? 
2. By what special apparatus do larger embryos take in oxygen? 
3. How is the oxygen distributed through the embryo’s body? 
