THE CYTOPLASM 41 



house built to contain the chromatic elements, and its achromatic ele- 

 ments (linin, etc.) were originally a part of the general cell-substance. 

 Moreover, as Carnoy points out, the house periodically goes to pieces 

 in the process of mitotic division, the chromatin afterward " building 

 for itself a new dwelling." 



3. Chemistry of the Nucleus 



The chemical nature of the various nuclear elements will be considered in Chapter 

 VII., and a brief statement will here suffice. The following classification of the 

 nuclear substances, proposed by Schwarz in 1887, has been widely accepted, though 

 open to criticism on various grounds. 



1. Chromatin. The chromatic substance (basichromatin) of the network and of 



those nucleoli known as net-knots or karyosomes. 



2. Linin. The achromatic network and the spindle fibres arising from it. 



3. Paralinin. The ground-substance. 



4. Pyrenin or Parachromatin. The inner mass of true nucleoli. 



5. Amphipyrenin. The substance of the nuclear membrane. 



Chromatin is probably identical with nuclein (p. 332), which is a compound of 

 nucleinic acid (a complex organic acid, rich in phosphorus) and albuminous sub- 

 stances. In certain cases (nuclei of spermatozoa, and probably also the chromo- 

 somes at the time of mitosis) the percentage of nucleinic acid is very large '(p. 333). 

 The linin is supposed to be composed of < plastin" a substance identified by 

 Reinke and Rodewald ('81) and probably a nucleo-albumin or a related substance. 

 '" Pyrenin " is related to plastin ; and Carnoy and Zacharias apply the latter word to 

 the nucleolar substance, while O. Hertwig calls it paranuclein. "Amphipyrenin" 

 has no very definite meaning; for the nuclear membrane sometimes appears to be of 

 the same nature as the linin, while in other cases it stains like chromatin. For cri- 

 tique of the staining reactions see page 334. 



D. THE CYTOPLASM 



It has long been recognized that in the unicellular forms the 

 cytoplasmic substance is often differentiated into two well-marked 

 zones : viz. an inner medullary substance or endoplasm in which the 

 nucleus lies, and an outer cortical substance or exoplasm (ectoplasm) 

 from which the more differentiated products of the cytoplasm, such 

 as cilia, trichocysts, and membrane, take their origin. Indications of 

 a similar differentiation are often shown in the tissue-cells of higher 

 plants and animals, 1 though it may take the form of a polar differen- 

 tiation of the cell-substance, or may be wholly wanting. Whether 

 the distinction is of fundamental importance remains to be seen ; but 

 it appears to be a general rule that the nucleus is surrounded by 



1 This fact was first pointed out in the tissue-cells of animals by Kupffer ('75), and its 

 importance has since been urged by Waldeyer, Reinke, and others. The cortical layer is 

 by Kupffer termed paraplasm, by Pfeffer hyaloplasm, by Pringsheim the Hautschicht. The 

 medullary zone is termed by Kupffer protoplasm, sensu strictu; by Strasburger, Korner- 

 plasma ; by Nageli, polioplasm. 



