54 INTRODUCTION 



to isolate nuclear elements, especially the chromatin, for analy- 

 tic 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, cholesterin, and inorganic 

 salts has not yet been worked out, except that Macallum l 

 has found that nuclei contain no chloride, as indicated by their 

 not staining with silver nitrate, and also no potassium. 2 



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. 3 Their chemical composition has not been 

 ascertained. Zacharias considers the nucleoli as composed of 

 nuclein well saturated with proteid, because of its staining reac- 

 tions 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 4 found that 

 they reacted for organic phosphorus microchemically, but less 

 strongly than did chromatin fibers. 



The nuclear membrane is an uncertain 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 con- 

 tinuous, 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 membrane of partially semipermeable charac- 

 ter, maintaining different conditions in nucleus and cytoplasm. 



Functionally the nucleus is the essential element of the cell ; 

 an isolated nucleus may be able to develop new cytoplasm, but 

 isolated cytoplasm soon disintegrates, although it may manifest 

 life for some time by movement and chemical activities. A 

 popular theory is that synthetic, constructive processes occur in 

 the nucleus or under the influence of its products, but to what 

 the nucleus owes these hypothetical powers is unexplained. 

 More tangible are the theories based upon the work of Spitzer, 

 Loeb, 5 Lillie 6 and others which show that the oxidative pro- 

 cesses of the cell depend upon the nucleus, hence portions of 

 the cell cut away from the nucleus undergo asphyxiation. As 



1 Proceedings of the Koyal Society, 1905 (76), 217. 



2 Jour, of Physiol., 1905 (32), 95. The reliability of the method used by 

 Macallum has been questioned by Tracy (Jour. Med. Besearch, 1906 (14), 447). 



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

 *Proc. of the Eoyal Society, 1898 (63), 467. 



5 " Studies in General Physiology," Chicago, 1905. 



6 American Journal of Physiology, 1902 (7), 412. 



