PRIMARY ORGANIZATION OF LIVING MATTER 27 



the chromatin. Nucleoh are therefore not to be confused with bunches 

 of chromatin, which have sometimes been called nucleoli. The nature 

 and function of the nucleolus, when it is present, are not understood. 

 Some biologists have regarded it as a waste product; others have held it 

 to be a reserve supply of materials used in cell division, since it dis- 

 appears during that process; and it has been regarded as a reserve food 

 supply for the nucleus. 



The Cytosome. — The body of a cell is seldom uniform in composition 

 but includes a number of different structures. The more common ones 

 are here described, though very few cells have all of them. At the surface 

 there may be a definite cell wall which is lifeless, not composed of proto- 

 plasm but secreted by the cell. It is very common in plants, where it is 

 composed mostly of cellulose, one of the principal components of wood. 

 Some animal cells have such a lifeless covering, but in them it is often 

 made of other materials. Sometimes the cell is covered by a much 

 thinner and more flexible coat, the pellicle, as are the cells of Fig. 17. 

 Beneath the cell wall, or at the surface of the cell if there is no other cover, 

 a somewhat firmer layer which may be called the cell membrane is formed 

 out of the protoplasm itself in about the same way that water forms a 

 film at its surface. 



Within the cytosome, plastids are common. In the higher plants 

 they are universal and are usually green. Some are of other colors, 

 as in fruits and flowers, and some are colorless. In animals, plastids are 

 found chiefly in certain classes of protozoa (one-celled animals) where 

 they are mostly colored. 



Vacuoles are vesicles of liquid enclosed in the protoplasm. They 

 may be permanent or temporary. In the protozoa, temporary vacuoles 

 are common. They usually either enclose bodies of food in process of 

 digestion, in which case they are called food vacuoles, or disappear at 

 intervals by ejecting their liquid contents through the surface layer of 

 protoplasm into the surrounding medium. The latter kind is called a 

 pulsating or contractile vacuole. In some cells a centrosphere is found, 

 usually near the nucleus. It is a mass of somewhat differentiated proto- 

 plasm, containing a minute body that stains deeply, the centrosome 

 or centriole. When present, the centrosphere takes a conspicuous though 

 probably unimportant part in cell division, as described in another 

 chapter. 



Structures known as mitochondria (Fig. 19) are found in many kinds 

 of cells, perhaps in all cells. They are of various shapes — rods, threads, 

 granules — and occur almost anywhere in the cytosome. Many conjectures 

 regarding their function have been made, but little is definitely known 

 regarding it. An object known as the Golgi apparatus, of various forms, 

 often a conspicuous network, occupies various positions, usually near the 



