PROTOPLASM AND CELLULAR ORGANIZATION 



21 



like the egg of a bird, are very large. The 

 large size of some egg cells is due chiefly to 

 the accumulation of an enormous quantity 

 of reserve food material, and not to the 

 protoplasm they contain. Cells differ in 

 shape (Fig. 43, p. 85); they may be col- 

 umnar, flat, spherical, stellate, or long and 

 thin. There are trillions of cells in a complex 

 animal; there are about 9.2 billion in the 

 gray matter of the human brain alone. On 

 the other hand, certain animals (proto- 



zoans) may consist of a single cell. The size 

 of an animal usually depends not upon the 

 size of the cells but upon the number. 



Structure of cells 



The nucleus and certain other bodies can 

 sometimes be seen in living cells when 

 viewed under the higher powers of a com- 

 pound microscope, but special preparation 

 is necessary to make visible most of the 



Vacuole 



fCenfrosphere 

 Centrosome < 



Nucleoplasm 



Chromatin 

 Nucleus — 

 Nucleolus 



Nuclear membrane 

 Golgi apparatus 



Mifochondria 



Fat droplet 

 Cytoplasm 



Cell membrane 



Figure 6. Diagram of a generalized animal cell, that is, one showing the structures found in 

 various animal cells; all these parts are not necessarily present in all cells. The shape of mito- 

 chondria may change, depending on the phase of cellular activity. 



structure. This is accomplished by treating 

 living cells with dyes or by killing and then 

 staining them. The electron microscope has 

 greatly increased our knowledge concerning 

 the smallest structures of the cell (Fig. 7). 

 A diagram of the structure of a stained 

 animal cell containing most of the bodies 



that may be observed in cells of various types 

 is presented in Fig. 6. The animal cell is 

 surrounded by a thin cell membrane. A 

 rigid cell wall outside of the limiting cell 

 membrane is characteristic of plant cells, but 

 is rare in animals. The most conspicuous 

 body in the cell is the nucleus. This is 



