210 THE MEANING OF EVOLUTION 
tadpole ancestor. Its limbs develop as little buds in- 
distinguishable from similar buds that would have 
formed fins for the fish or wings for the bird. 
Around the embryo there forms a sac, the amnion 
filled with a fluid which serves to protect the young 
mammals exactly as the growing chick was protected. 
Under the forming creature there hangs a small but 
empty yolksac. This is an actual remnant, a reminder 
of the past, when the eggs of the mammals were also 
packed with yolk and the growing embryo secured 
its nourishment exactly as does the maturing chick. 
But a new method has been provided for the mam- 
mal, and consequently the yolksac, though it has not 
entirely disappeared, has no nutritive content for the 
growth of the embryo. 
The allantois of the chick now gains a new devel- 
opment and an altered function. In the case of the 
chick it floats against the shell of the egg and absorbs 
oxygen through the shell. Inside the body of the 
mammal this is impossible, because the air is too far 
away. No shell is formed about the egg because it 
is not to be laid. The tube of the parent’s body in 
which the egg lies becomes thickened at the point of 
contact with the egg. It grows spongy and full of 
blood vessels. Meanwhile the allantois is also grow- 
ing spongy. These two tissues are so closely pressed 
against each other that the blood vessels of the trans- 
