Flat epithelial cells 



^ \ "^ >-A:< 



Columnar epithelial cells 



Unstriped muscle cells 



Dendrites 



TYPES OF ANIMAL CELLS 



Bone 

 cells ~ 



Shapeless ameba cells 



Cells 

 containing 

 fat globules 



Axon 

 Nerve cell, or neuron 



Terminal .--- •O"'^?^ 1 

 branches' 



however, consist of very few cells or, like the ameba, of a single cell. Bac- 

 teria, of which everybody hears a great deal, are one-celled plants. So are 

 many algae, for example the "green-slime", which lives on the shady side of 

 trees or on damp shingles. But every plant or animal, whether it consists of 

 a single protoplasm unit or of many millions of cells, starts out as a single 

 cell. Among the one-celled organisms, a new individual originates by a 

 comparatively simple division of a parent cell — one cell becomes two! The 

 nucleus divides into two equal parts, and then the rest of the protoplasm 

 divides. Thus two distinct cells result (see illustration, p. 10). 



In many-celled animals the body grows as cells increase in size. When 

 a particular cell reaches its full size, it may divide into two. The nucleus 

 splits first and then the rest of the protoplasm. A new individual usually 

 arises from special cells which become separated from the parent body (see 

 Chapter 19). 



Protoplasm Is Fundamental In the one-celled ameba, as we have 

 seen, the single bit of protoplasm carries on all the life activities. It grows, 

 it moves, it reproduces, and so on. Yet in the larger plants and animals, 

 those having many kinds of cells and millions of each kind, the protoplasm 

 of each cell carries on the same fundamental activities. However different 

 a bone cell may be from a brain cell, or a tree cell from a dog cell, the 

 protoplasm in all cases is irritable, it can grow, it can move, and at some 

 stage of its life it can reproduce itself. 



The many different kinds of plants and animals, with their peculiar 

 forms and organs and many kinds of activities, are a constant source of 

 wonder. Yet they all apparently arise from protoplasm, which is always the 



25 



