THE CELL 311 



ally spherical masses and, therefore, distinguishable from the 

 chromatin. But a better means of distinguishing between 

 these is given by the fact that they are not stained by the 

 same dyes. 



675. The linin is a part of the nucleus which does not stain 

 at all by ordinary methods. It also assumes various forms, 

 but when most evident it is as a system of fibres, or a network 

 of threads by which the other elements of the nucleus are 

 bound together. 



676. The interstices of the nucleus are filled with a nuclear 

 sap. 



677. There is one other structure in the cell which must be 

 mentioned, though there is some doubt whether it should be 

 classed with the cytoplasmic or nucleoplasmic structures. This 

 is the centrosome. It is generally found in the cytoplasm close 

 by the side of the nucleus, but sometimes it is far removed, and 

 again it seems to be enclosed by the nuclear membrane. The 

 centrosome is excessively small, scarcely more than a point, 

 even with the highest powers of the microscope, but it may be 

 stained by certain methods, and is further distinguished from 

 other minute protoplasmic structures by the radial arrange- 

 ment of the surrounding protoplasm, for w^hich it forms a 

 centre. Something concerning its significance will appear in 

 the discussion of cell division. 



678. In this brief description of the protoplasm, only the 

 most important constant structures have been mentioned. 

 These have each their optical, physical, and chemical peculi- 

 arities. The list of substances which have been recognized 

 might be greatly extended and yet, because of our imperfect 

 instruments and methods we are far from having made a com- 

 plete analysis of the protoplasm. It seems probable that fur- 

 ther investigation will show that among the innumerable mi- 

 nute particles which at present are indistinguishable one from 

 another are many chemically and otherwise distinguishable kinds. 



