Protozoa. 303 



adhering to each other, and that it divides again, making 

 four parts, and each of these divides, making eight parts 

 remaining in one mass. It is easy to see how one might 

 attend to the moving, another to the work of feeling, a 

 third to eating and digesting, a fourth to breathing, a fifth 

 to the work of dividing for the spread of the species, while 

 the other three became more or less flattened and spread 

 out over the others to protect them. This will serve to 

 give an idea of what actually takes place when the egg 

 cell of a metazoan has, by division, become a many-celled 

 mass. Each cell has primarily all the characteristics 

 common to an amoeba, that is, they all have power to 

 move, eat, digest, feel, breathe, divide. But such a large 

 mass would be unwieldy unless it had some support ; 

 hence some cells may, to the advantage of the whole, 

 become harder and stronger to hold the soft cells in place. 

 In this now heavier mass there is more danger of mechani- 

 cal injury to the outside, and the outside layer by harden- 

 ing will serve to protect the inner, more delicate cells from 

 harm. If the outside cells harden, the animal will no 

 longer be able so well to absorb oxygen for the interior 

 cells, nor can it now take in food at any point. Special 

 arrangement must be made to take oxygen and food into 

 the interior, where soft cells can do the work of preparing 

 it for use in the body. These inner cells have now more 

 work than before, for they must prepare the material for 

 building and maintaining the cells that have given up their 

 power of digesting that they might fit themselves for pro- 

 tection. In proportion as any given cell devotes itself to 

 one kind of work, it must lose more or less the ability to do 

 the other kinds of work that it primarily could do. This 

 is what is meant by physiological division of labor. All 

 the cells resulting from the division of a metazoan egg 



