542 STRUCTURE OF CELLS chap. 



are in focus at one time (see e.g. p. 315) : 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 

 cell-membrane of extreme tenuity : 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. 136, A, nu.m). 

 The achromatin is a homogeneous semi-fluid substance 

 which forms the groundwork of the nucleus : it resembles 

 the clear cell-protoplasm in its general characters, amongst 

 other things in being almost unaffected by dyes. The 

 chromatin (chr) takes the form of a network or sponge-work 

 of very variable form, and is distinguished from all other 

 constituents of the cell by its strong affinity for aniline and 

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

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

 structures, the nucleoli {nu'), occur -in the nucleus either 

 connected with the network or lying freely in its meshes : 

 they also have a strong affinity for dyes although they 

 often differ considerably from the chromatin in their micro- 

 chemical reactions. 



In the body of some cells and possibly of all there is 



