34^: STRUCTURE OF CELLS CHAr. 



are in locus at one time (sec I'.g: p. 315) : by altering the 

 tocLis 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 b\ a delicate network. In the epithelial 

 cells of animals the protoplasm is bounded externally by a 

 ceU-ineiiibraiie of extreme tenuity : in amceboid cells the 

 ectoplasm or transparent, non-granular portion of the cell 

 consists of clear protoplasm only, the granular endoplasni 

 alone possessing the sponge-work. 



IL is quilc })ossiblc LhaL the relicuiar character ni the cell tua)' be 

 merely the optical expression of an extensive but minute vaciiolation, 

 or may be due to the presence of innumerable minute granules lievclopeil 

 in the prt.itoplasm as products of metabolism. 



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

 in a delicate niickai- inemhraiie (Fig. 136, .\, luuii). 

 'I'he acliroiiiatin 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 

 chroinatin {clir) 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 (l\es (compare Part I., Chapters \\\ and \TII). 

 Krequentl)-, as we jiavc seen, one or more minute globular 

 structures, the iiiickoli {iiii"), occur -in the nucleus either 

 connected nith the network or King freely in its meshes: 

 they also ha\e a strong affinity fijr d)es although they 

 olteii iJiffer ronsiderably from the chromatin in their micro- 

 cliemii.al reactions. 



In the liijdy of some cells and pos.silily of all there is 



