24 FOUNDATIONS OF BIOLOGY 



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 smooth muscle, 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 units more frequently present 

 symmetrical, angular outlines. (Figs. 7, 42.) 



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

 regarded as the more important part, and its protoplasmic 

 contents, if observed at all, were considered as only of second- 

 ary importance, if not a waste product. Now we recognize 

 many cells which are merely naked masses of protoplasm, 

 such as certain types of blood cells. In other words, the 

 protoplasm is the essential living part the cell wall 

 frequently being a non-living accessory which more or less 

 sharply delineates one unit mass of protoplasm from another 

 and lends rigidity and form to the group of cells as a whole. 



1. Cytoplasm 



The protoplasm of all typical cells is differentiated into 

 two parts: the CYTOPLASM, or general groundwork which 

 makes up the bulk of the cell; and the NUCLEUS, a restricted, 

 clearly defined area, usually situated near the center of the 

 cytoplasmic mass. 



The cytoplasm may be considered the more generalized 

 protoplasm of the cell, and its appearance and other character- 

 istics are those which have been outlined in our discussion of 

 protoplasm. With that in mind, for the sake of definiteness, 



