112 



HOW ANIMALS DEVELOP 



ment seems to be very important for the development 

 of a really satisfactory joint. 



The movements which developing embryos make 

 are not the only functional factors which have im- 



FiG. 28. — Differences in the guts of tadpoles fed {a) mainly 

 on vegetables, {b) exclusively on meat. (From Durken.) 



portant effects on development. Sometimes a develop- 

 ing structure is put under tension because it is pulled 

 out by its rapidly growing surroundings. Such ten- 

 sions are probably very important, because if any- 

 thing is stretched, any minute particles within it 

 tend to become orientated parallel to the direction 

 of stretching. Now it seems that the spaces in the 

 embryo are sometimes filled with a jelly-like sub- 

 stance which is made up of little elongated particles 

 which get orientated in various ways under the 



