CHAP, xii STRUCTURE OF CELLS 559 



are in focus at one time (see e.g. Fig. 71 B) : by altering the 

 focus we view the object at successive depths, each view 

 being practically a slice parallel to the lenses of the 

 instrument. This being this case, protoplasm presents the 

 microscopic appearance of a clear or slightly granular 

 matrix traversed by a delicate network. In the epithelial 

 cells of animals the protoplasm is bounded externally by 

 a delicate cell-membrane : in amoeboid cells the ectoplasm, 

 or transparent, non-granular portion of the cell consists 

 of clear protoplasm only, the granular endoplasm alone 

 possessing the sponge-work. 



It is quite possible that the reticular character of the cell may be 

 merely the optical expression of an extensive but minute vacuolation, 

 or may be due to the presence of innumerable minute granules 

 developed in the protoplasm as products of metabolism. 



The nucleus is spherical or oval in form, and is enclosed 

 in a delicate nuclear membrane (Fig. 146 A). Its contents 

 consist of a homogeneous semi-fluid substance throughout 

 which an extremely fine network of threads extend : in this 

 framework are imbedded granules of chromatin (chr\ which 

 are distinguished by their strong affinity for aniline and 

 other dyes (compare e.g. Part I., Chapters VII and VIII). 

 Frequently, as we have seen, one or more minute globular 

 structures, the nucleoli (kk\ occur in the nucleus and 

 are to be looked upon as transient structures and not as 

 actual living parts of the cell : they also have a strong 

 affinity for dyes, although often differing considerably from 

 the chromatin in their micro-chemical reactions. 



Close to the nucleus a small globular body, called the 

 centrosphere (cspti), enclosing a minute particle or centrosome, 

 is present in the cell ; it is inconspicuous and often difficult 

 to recognise, but probably occurs in all cells which are 

 capable of division. 



