6 INTRODUCTION. 



in a drop of water, cover with a coverslip, and examine under 

 a quarter-inch objective. Many cells will be seen, some of 

 them showing nuclei. -Compare these cells with the accom- 

 panying figure. 



In the active tissues the cells are alive, and each cell 

 may be considered as like the Ameba, a little mass of living 

 jelly-like substance called Protoplasm. Within this is a small, 

 rounded part called the Nucleus. Most of the cells of the 

 body differ from the Ameba in having a distinct outer cover- 

 ing or Cell-wall. A grape serves very well to show what a 

 cell is like. Now, the whole body is built up chiefly of such 

 little cells, few of them large enough to be seen by the naked 

 eye. Although the cells are closely 

 packed together, each cell leads, in 

 one sense, an independent life. I>ut 

 all work together to maintain the life 



of the body as a whole. The cell 

 Nucleus. ig nke the individual in 



Fig. 1. Epithelial Cells from 



the inside of the Cheek. a community. Each lives primarily 

 for itself, yet all work together for 



the good of the whole. Each has its own kind of work to 

 do, as have the individuals of the community. 



The Physiological Division of Labor. We are aware 

 of the advantages of division of labor in a community. If 

 each person learns to do one thing well, all work economically 

 together for the common good, time is saved, and better goods 

 of all sorts are produced. As a rule, civilization is at a low 

 ebb where every individual is a " jack-of -all-trades " and mas- 

 ter of none. In the body there is a division of labor similar 

 to that of a community. Each of the different organs has its 

 own work to do, and all work together for the common wel- 

 fare. Each of the tissues is composed of cells characteristic 

 of that tissue. T?he cells of one tissue have certain properties 



