INTRODUCTION 



scientific study has shown to be the carriers of hereditary charac- 

 teristics, but which may yet share the honors with unknown units. 

 Chemical or physical agents may combine to determine the activity 

 of these granules. 



Cey^fro/ bodies Coenfr-osomc) 



_ '^/" 



PJasmosome or true nucleolus f ■^Y_Mf-^'fc)te^.;,^^/(_' A 



Chromatin ^Z t^^W^^^M\'^^-A\ 



Linm _ ^ i-(^L-v<^P'_:.:c*--i^ 



Koryosome or chromatin- nucleoJus ' ' ^ ' 



vr/osom es 



:^^'^ I "-^ ,% V^V-'-Trive wall or membrane 



^^^m^^i^&i'V" ^'^■^■^'^^ i^etoplosmic or 



paroplosfic boc/i&s 



Fig. I. Diagram of a generalized cell. (After Wilson, The Cell in Development and 

 Heredity. Courtesy of The Macmillan Co.) 



Cell Membrane and Cell Wall. — The cell membrane protects 

 the cytoplasm from many physical and some chemical injuries. 

 It is, however, only semi-permeable, and admits some solvents, but 

 retains the colloids of its own protoplasm, and excludes foreign ones. 



On account of the tendency for water to pass or diffuse from a 

 liquid with low concentration of soluble substance, through a per- 

 meable membrane to a liquid with higher concentration, we find that 

 the energy or osmotic pressure varies with the degree of similarity 

 of the concentrations. In discussing two solutions, we refer to 

 their tonicity. For example, a solution losing water through a 

 membrane would be hypotonic, while the one that gained the water 

 would be called hypertonic. Two solutions in osmotic equilibrium 

 are called isotonic. 



The cell membrane may have a covering of some resistant 

 material like chitin or hardened gelatin in the animals, or of woody 

 cellulose in the plants. This covering is called the cell wall. 



Cytoplasm. — The most conspicuous structure in the cytoplasm, 

 aside from the nucleus and certain large vesicles and plastids, is the 

 centrosome, or central body (consult Figure i), which we find acting 

 as the division center for the aster in mitotic division. (See page 

 500.) No centrosome is present in the cells of flowering plants. 



Cytoplasmic Granules. — Granules of various kinds are scattered 

 throughout the cytoplasm, suspended in the clear, viscid "hyalo- 

 plasm." The relatively large yolk granules occur as solids, semi- 

 solids or as liquid drops. Fat and glycogen appear as cell inclusions. 



