CHAP, xii STRUCTURE OF CELLS 559 



Fig. 71, B, and p. 323) : by altering the focus we view the 

 object at successive depths, each view being practically 

 a slice parallel to the surface of the lenses. This being 

 the case, protoplasm presents the microscopic appearance 

 of a clear or slightly granular matrix traversed by a 

 delicate network (Fig. 146). In the epithelial cells of 

 animals the protoplasm is bounded externally by a very 

 delicate cell-membrane : in many amoeboid cells the 

 ectoplasm, or transparent, non-granular portion of the 



nucl. 



cyt. 



FIG. 146 Diagram of a typical cell. 



ch. g. chromatin granules ; c. m. cell-membrane ; c. s. centrosomes lying in centro- 

 sphere ; cyt. cytoplasm ; n. m. nuclear membrane ; nu. nucleus ; nufl. nucleolus. 

 (From Dendy's Outlines of Evolutionary Biology.) 



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 en- 

 closed in a delicate nuclear membrane (Figs. 146 and 147, 

 A). Its contents (nudeoplasm) consist of a homogeneous 

 semi-fluid substance throughout which an extremely 



