44 



THE CELL 



system. In multicellular organisms the element of pressure has 

 frequently much to do with determining the shape of the cell. Also 



as most cells are soft and pliable their shapes frequently change in 

 response to changes in function or in environment. 



In most cells the structure of the protoplasm is not uniform 

 throughout the cell but is differentiated to form certain special cell 

 structures. Thus in most cells a portion of the protoplasm is specially 

 modified to form a body known as the nucleus. Also at the sur- 

 face of the cell there is usually some differentiation of the protoplasm 

 to form a more or less distinct cell wall or cell membrane. An 

 ,; actively multiplying cell con- 



tains a minute structure 

 associated with the reproduc- 

 tive function and known as 

 the centrosome. 



A typical cell thus con- 

 sists of the following struc- 

 tures (Fig. i): (i) The cell 

 body; (2) the cell membrane; 

 (3) the nucleus; (4) the cen- 

 trosome. Of these only the 

 cell body with its modified 

 surface cytoplasm or cell 

 membrane is present in all 

 cells. A few cells contain, 

 in their fully developed condi- 

 tion, no nuclei. In many 

 mature cells it is impossible 



Fig. 2. — Diagram Illustrating Theories of 

 Protoplasmic Structure, a, Fibrillar theory; 

 b, granule theory; c, "foam" theory. (The 

 general structure of cell body and nucleus cor- 

 responds.) 



to distinguish a centrosome. 



I. The Cell Body. — This consists of a viscid, colorless, semi-fluid 

 substance, belonging to the general class of albumens and known as 

 protoplasm. It is alkaline in reaction and of complex chemical 

 composition containing the elements carbon, hydrogen, oxygen and 

 nitrogen in quite constant proportions, and smaller variable quanti- 

 ties of phosphorus, sulphur, iron and other substances. It contains 

 a peculiar nitrogenous proteid, plastin. Structurally it can be 

 differentiated into a formed element, spongioplasm, and a homo- 

 geneous element hyaloplasm. Distributed along the spongioplastic 

 network are minute granules, microsomes. The exact relations which 



