130 



FUNDAMENTALS OF STRUCTURE AND FUNCTION 



Xell membrane 



Cytoplasm 



..Cenfrosomes 

 ...Nucleolus 



Nucleus 

 Plasiid 



Vacuole 



Diagram of a typical animal cell. 



to the outer part of the cell, while the vacuoles form large sap cavities 

 within the cell. Although Golgi bodies appear much less stable and 

 more changeable in form than plastids, they are found in many kinds 

 of plant and animal cells. Fibrils of various kinds, such as those 



seen in a muscle cell, are frequently 

 found. In plants the cell wall, a 

 delicate but rigid, secreted cellulose 

 covering, is lined with a delicate 

 living membrane which separates 

 the living stuff from the cell wall. 

 At one point can be found a slightly 

 denser jellylike part of the proto- 

 plasm called the nucleus. Both 

 the vacuoles and the nucleus are 

 separated from the cytoplasm by 

 delicate membranes. In many but 

 not all nuclei, dense, dark-staining 

 nucleoli appear. While their func- 

 tion is not clearly understood, they 

 generally break up and disappear 

 during cell division. The nucleus proper is a vital, definite part of 

 every living cell and is of great importance in cell division which 

 must take place if a many-celled organism is to grow in size, for 

 growth takes place by an increase in the number of cells, not in the 

 size of the cells. The nucleus is filled with nuclear sap, in which 

 is found a network of linin fibers. On these fibers are scattered 

 numerous granules of chromatin. This material, which as we will 

 see later forms chromosomes, is of the greatest importance, as through 

 it plants and animals are able to pass on to successive generations 

 their inheritable qualities. 



Chemical Organization of Living Matter 



A dozen or more of the ninety-odd elements recognized by the 

 chemist are found in living protoplasm, — carbon, hydrogen, oxy- 

 gen, and nitrogen comprising the greatest bulk. These elements also 

 form the basis of our so-called organic foodstuffs, which are called 

 proteins, carbohydrates, and fats. The two latter groups of sub- 

 stances are made up of carbon, hydrogen, and oxygen, while the pro- 

 teins have the element nitrogen added to their constitution, along 

 with sulphur, phosphorus and sometimes iron. In a simple carbo- 



