THE CYTOPLASM 2g 



3. l\iralinin. The ground-substance. 



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



5. Ainphipyrenin. The substance of the nuclear membrane. 



Chromatin is probably identical with nuclein (p. 240), which is a compound of 

 nucleic acid (a complex organic acid, rich in phosphorus) and albumin. In certain 

 cases (nuclei of spermatozoa, and probably also the chromosomes at the time of 

 mitosis), chromatin may be composed of nearly pure nucleic acid. The linin is 

 probably composed of "plastin," a substance similar to nuclein, but containing a 

 lower percentage of phosphorus, and either belonging to the nucleo-proteids or 

 approaching them. It is nearly related with the substance of the cyto-reticulum. 

 Pyrenin consists of a plastin-substance which stains like linin. Amphipyrenin is 

 probably identical with linin, since the nuclear membrane is probably a condensed 

 portion of the general reticulum which forms the boundary between the intra- and 

 extra- nuclear networks. It should be borne in mind, however, that the membrane 

 often has an inner chromatic layer composed of chromatin. 



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 differ- 

 entiation 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 

 protoplasm of relatively slight differentiation, while the more highly 

 differentiated products of cell-activity are laid down in the more 

 peripheral region of the cell, either in the cortical zone or at one 

 end of the cell. 2 This "fact is full of meaning, not only because it is 

 an expression of the adaptation of the cell to its external environment, 

 but also because of its bearing on the problems of nutrition. 3 For if, 

 as we shall see reason to conclude in Chapter VII., the nucleus be 

 immediately concerned with synthetic metabolism, we should expect 

 to find the immediate and less differentiated products of its action in 

 its neighbourhood, and on the whole the facts bear out this view. 



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 strict u ; by Strasburger Korner- 

 plasma, by Nageli polioplasm. 



2 Cf. p. 38. 



3 See Kupffer ('90), pp. 473-47- 



