HOW ANIMALS DEVELOP 



CHAPTER I 

 INTRODUCTION 



The Development of Animal Organization 



Living animals are constantly on the move. It is 

 one of the most characteristic things about them. 

 Often we can see them running about, breathing, 

 catching food and eating it, and so on. If we look 

 closer we find that an animal is made up of different 

 organs, and in all of them there is something going 

 on all the time. On an even smaller scale, the organs 

 are built out of cells, little lumps of living matter, 

 each containing a special kernel or nucleus. And each 

 cell is always full of activity. In plants the living 

 jelly streams slowly about from one side of the cell 

 to the other: in animal cells we cannot usually see 

 any movement, but nevertheless there are incessant 

 chemical actions and reactions. The cell absorbs 

 oxygen and other substances from outside, performs 

 many complicated chemical operations with them, 

 and pours out again into its surroundings the by- 

 products for which it has no use. 



In a living organism these changes are not isolated 

 but are adjusted to one another so that the right 

 operations are carried out to produce the right 

 quantities of the various products. It is because 



