CELLULAR ORGANIZATION OF LIFE 



9 



A. The Cell 



With the diversity of gross structure of animals and plants in 

 mind, one is not surprised that there are considerable, even great, 

 variations in their component cells. In fact, the characteristics of 

 an organism or part of an organism are determined by those of 

 the cells. But there are certain fundamental cell characters which 

 are common to all cells — by virtue of which they are cells — and 

 it is important to emphasize these. (Fig. 5.) 



In its simplest form a cell is a small, more or less spherical mass 

 of protoplasm. Such are the eggs of various animals and the com- 



Fig. 5. — Diagram of a cell shown (A) as a solid body, and (B) in 

 section, through plane x-y, as it appears under the microscope 



plete body of some of the lowest plants and animals. Cells forming 

 the units of multicellular organisms, however, frequently exhibit 

 more or less hexagonal surfaces on account of stresses and strains 

 incident to their position among other cells; while specializations 

 and differentiations, for one purpose or another, produce forms 

 which are characteristic of different parts of the organism, as, for 

 example, the long spindle-shaped cells of certain muscles, and the 

 widely branching cells of parts of the brains of animals. Broadly 

 speaking, the greater diversity of cell form is found in animals, 

 while in plants, owing to the more general presence of rigid cell 

 walls about the protoplasm, the cells more frequently present 

 symmetrical, angular outlines. (Figs. 6, 7.) 



The term cell is a relic of the time when the cell wall was re- 

 garded as the most important part, and its protoplasmic contents, 

 if observed at all, were considered as only of secondary importance, 

 if not waste material. Now we recognize many cells which are 



