48 



KERATIN AND KERATINIZATION 



The nuclear membrane may be considered for the purposes of classifica- 

 tion among the cytoplasmic membranes. It consists of two surfaces, 

 whose distance apart is rather variable (500-1000 A), and which touch at 

 intervals to give the impression of a circular pore (Plate 4C). These pores 

 may form a regular pattern over the surfaces of some nuclei and are held 

 by some authors (Watson, 1954) to be genuine pathways permitting 

 nuclear-cytoplasmic interchanges at a macromolecular level. 



Fig. 23. Nomenclature of cytoplasmic membranes. 



* Occasionally Porter and Palade prefer the less restricted term 

 " reticulum " to cover the entire system of membranes. 



t For these the name S-cytomembranes has been proposed (Schulz 

 et al., 1958). 



The nucleus 



Within the volume delimited by the nuclear membrane there are no 

 membrane enclosed objects. One or more dense aggregates of particles 

 appear as the images of the basophilic nucleolus of light microscopy (see 

 p. 80). At the appropriate phases of cell division the nuclear membrane 

 dissolves and chromosomes may appear as denser aggregations of particulate 

 or finely-fibrous material. 



Differentia 



Under this name we gather a variety of cell products, which seem 

 distinct from the vital synthetic and respiratory machinery of the cell 

 and represent rather the end results of a specialized path of synthesis. 

 Their presence often gives the cell (and tissue) its characteristic appear- 



