2IO 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- 



