Introduction 



ii 



Every perfect cell consists of a cell-body and a nucleus. 



In brief, then, a cell is a nucleated mass of protoplasm endowed 



with the attributes of life. In this sense the term " cell " is 



FIG. 2. Amoeboid Movement of a Human White Blood Corpuscle. 

 (Showing various phases of movement.) 



now used. Every tissue of the human body is formed 

 through the agency of protoplasmic cells. 



17. The Differentiation of Cells. The simplest forms of 

 animal life are organisms consisting of only one cell. Thus, 

 the amoeba begins its life as a cell split off from its parent. 

 This divides in its turn, and each half is a complete 

 amoeba. Organisms a little higher 



than the amoeba consist of many 

 cells. As we ascend in the animal 

 scale the cells adapt themselves to 

 perform certain definite functions 

 and a division of labor occurs. 

 Technically, the cells are said to 

 undergo differentiation. 



Cells having similar shape and func- 

 tion are grouped to form tissues, and 

 tissues are grouped to form organs. 



18. Cells and the Human Organism. If the human body 

 be studied in its first stage of development, it is found to 

 originate from a single nucleated cell. From this original 

 cell, by growth and development, the body, with all its vari- 

 ous tissues, is built up. Many fully formed organs, like the 

 liver, consist chiefly of cells. Sometimes cells are modified 



FIG. 3. Nerve Cells from 

 the Gray Matter of the 

 Cerebellum. 

 (Magnified 260 diameters.) 



