CELL STRUCTURE 41 



it is possible to isolate nuclear elements, especially the chromatin, for 

 analytic purposes, and it has been demonstrated by this means also 

 that nuclein is the chief constituent of the staining elements. The 

 distribution in the nucleus of the other primary constituents of the 

 cytoplasm, such as lecithin, cholesterol, and inorganic salts has not 

 yet been worked out, except that Macalluni"''^ found that nuclei contain 

 no chloride, as indicated by their not staining with silver nitrate, and 

 also no potassium, ^^ but the chromatin contains firmly bound iron. 



Nucleoli, which not all varieties of nuclei possess, differ from the'other nuclear 

 structures in having an affinity for acid rather than for basic dyes," at least in fixed 

 tissues. Their chemical composition lias not been ascertained. Zacharias con- 

 siders the nucleoli as composed of nuclein well saturated with protein, because of 

 its staining reactions and its relative insolubility in alkalies, and classes it with 

 plastin or linin, which forms the achromatic part of the nucleus and is also present 

 in the cytoplasm. Macallum'* found that they reacted for organic phosphorus 

 microchemically, but less strongly than did chromatin fibers. 



The- nuclear membrane is an imcertain structure, at times dense and staining 

 as if formed of a layer of chromatin, in other cells staining like the cytoplasm with 

 which it seems to be continuous, in most cells disappearing during karyokinesis, 

 and in some protozoa being entirely absent. Naturally the composition of the 

 nuclear membrane is unknown, but it is probable that it acts as a diffusion mern- 

 brane of partially semipermeable character, maintaining different conditions in 

 nucleus and cytoplasm. 



Functionall}^ the nucleus is the essential element of the cell; an 

 isolated nucleus with but a minimum of cytoplasm may be able to de- 

 velop new cytoplasm, but isolated cytoplasm soon disintegrates, al- 

 though it may manifest Hfe for some time by movement and chemical 

 activities. It has been frequently suggested that the nucleus controls 

 oxidative processes, and there is some microchemical evidence for 

 this.^^ Lynch-^" calls attention to the improbability that the part of 

 the cell most removed from the oxygen should be the organ of oxida- 

 tion, and finds evidence that the function of the nucleus is that of 

 organic synthesis. An enucleated cell may move, respire, digest, 

 respond to stimuli and exhibit any activity which is dependent solely 

 upon catabolic or destructive processes of protoplasm. The group of 

 phenomena which it never shows are those of growth, regeneration and 

 division, i. e., those depending on synthetic activities. 



It should be mentioned that certain cells, such as bacteria and algse, 

 'seem to have no true nuclei, but Macallum^^ found that the forms he 

 examined gave reactions for phosphorus and iron in a similar way 

 to the nucleoproteins of a nucleus, suggesting that in such cells the 

 nuclear elements are diffused through the cell rather than differen- 

 tiated. To* quote Wilson: "The term 'nucleus' and 'cell body' 



35 Proceedings of the Roval Society, 1905 (76), 217. 



se Jour, of Physiol., 1905 (32), 95. 



3^ Nucleoli of nerve-cells are an exception, being basophilic. 



=58 Proc. of the Roval Societv, 1898 (63), 467. 



33 See Osterhaut, Science, 1917 (46), 367. 



« Amer. Jour. Physiol., 1919 (48), 258. 



*^ "Studies from the University of Toronto," 1900. 



