STRUCTURE OF A TYPICAL CELL 135 



rounded area of cytoplasm, the centros'phere encloses one or two 

 small granules, the centrioles or centrosomes. The centrosphere 

 and centrioles are conspicuous structures during mitotic cell divi- 

 sion (c/. p. 137). A number of bodies found more or less com- 

 monly in the ground substance of the cj^toplasm are grouped 

 under the name of cytoplasmic inclusions. Mitochondria are the 

 most universal members of this group. These are small gran- 

 ules, isolated or arranged in rows; they are typically scattered 

 throughout the cytosome but may be more numerous in some 

 regions. Golgi elements have been observed in many cells. Fatty 

 products of metabolism are stored as larger or smaller drops in 

 the majority of cell bodies. Yolk, built up in the cytoplasm, is 

 stored in the form of j'olk plates or spheres in egg cells. In 

 many cases, vesicles, filled with solutions of unknown compo- 

 sition, are present. When the contents of these vesicles are lost 

 they may appear as vacuoles. Secretions produced in gland cells 

 are stored as secretion granules until they are passed from the cells. 

 Granules of pigment characterize many cells. These cytoplasmic 

 inclusions constitute a rather diversified group, and full under- 

 standing of their origin and function is dependent on information, 

 much of which is yet to be gained, concerning the metabolism of the 

 cell. 



The Nucleus. — The nucleus, which is usually round and cen- 

 trally located, is everywhere surrounded by the cytosome. It is 

 separated from the cjiioplasm b}^ a continuous bounding mem- 

 brane, the nuclear memhrane, and typically exhibits a fine frame- 

 work known as the linin net. Scattered on the linin are fine 

 granules which are called chromatin, because with some stains 

 they color very intensely. Those chromatin granules are fre- 

 quently aggregated on the nuclear framework to form net knots. 

 Chromatin is the material that has been shown to be closely 

 correlated with the mechanism of heredity. Further description 

 of its behavior will be given in the accounts of cell division and 

 maturation. Lying in the meshes of the linin net are found one 

 or more rounded bodies, the nucleoli or plasmasomes. Nucleoli 

 have been interpreted as waste products of nuclear metabolism, 

 but a growing mass of evidence seems to indicate their more 

 fundamental importance as perhaps temporary storage products 

 of such metabolism. The nuclear components described above 

 are embedded in the ground substance of yiucleoplasm. 



