• THE INTERPHASE NUCLEUS 



and so the difficulty of demonstrating their presence by cytochemical 

 methods is greatly increased. Cohen^^^ has shown that the hetero- 

 chromatin in the interphase nuclei of the plants Allium and Phaseolus 

 responds to the powerful lipoid colorant Sudan black (Lison^^^), the 

 use of which in nuclear cytology should be extended. Gates^^' denied 

 that lipoids were present in the nucleolus; in some marine eggs, however, 

 Shinke and Shigenaga^^^ and Mensinkai^^^ report their presence 

 while Serra^^^ states that in the nucleolus of snail oocytes there are 

 'peripheral inclusions which give a coloration of middle intensity with 

 BZL blue'. It seems therefore that both in mitochondria and the nu- 

 cleoli, ribonucleic acid is associated with lipoproteins, though the 

 proportion of each constituent in the two types of structure are probably 

 widely different. The possible functions of the phospholipine within 

 the cell have been discussed by Claude. ^^^ 



nuclear membrane 



The debates on the validity of the detail seen in the fixed and stained 

 cell have included each of its main constituents, the nuclear membrane 

 among them. Pischinger,^^^ for instance, argued that this was an 

 artefact, since its prominence in sections of mouse liver varied with the 

 fixative used; furthermore he was unable to identify it in the unfixed 

 cell. First, therefore, we must consider the question whether the nuclear 

 membrane has a real existence. 



It must be admitted that the presence of an interface between nucleus 

 and cytoplasm does not necessarily prove that a physical membrane 

 is there, for Bungenberg de Jong^^^ has shown how two aqueous 

 colloidal solutions may be immiscible, if one of them is a .coacervate 

 (p 41). The argument that a nuclear membrane is not visible in life 

 is unlikely to survive the introduction of the phase contrast microscope. 

 The phase photomicrographs of living intermitotic cells in culture 

 (Plate I(i)) show how clearly this structure stands out, particularly 

 where nuclear sap and the adjacent cytoplasm are of about the same 

 refractive index. A vacuole within the cytoplasm has no such definite 

 border. In binucleate cells, the two nuclei are usually in contact, and 

 the apposed membranes usually appear thicker than the single layer 

 elsewhere on the circumference of the nucleus. Such cells have been 

 observed for several hours with their nuclei in apposition. In the divid- 

 ing cell, the disappearance of the membrane at the end of prophase is 

 immediately noticeable as the nuclear area then suddenly contracts. 

 Even before these particular observations were available, the evidence 

 from micro-dissection studies (Chambers and Fell^^^) had already pro- 

 vided a clear proof of the presence of this structure within the cells of 

 tissue cultures. 



51 



