THE INTERPHASE NUCLEUS 



treated with distilled water, all structure within the nucleus is dissolved, 

 and that this change is reversible when the culture is returned to a 

 normal saline medium. The nucleoli then reappear, though not in their 

 original outline. Hughes^^ observed that the nucleoli of cells in chick 

 tissue cultures are rapidly affected by treatment with the nucleoside 

 adenosine (Plate II (5)), or the purine analogue benzimidazole. These 

 substances cause the nucleoli to break up into granules which may 

 ultimately be scattered throughout the nucleus. This effect again is 

 readily reversed by washing the culture with fresh physiological saline. 



Coacervation 



It has been suggested that the nucleolus is of the nature of a co- 

 acervate (Hygen,^^ Ehrenberg,^* Duryee^^). Under certain circum- 

 stances, it is possible for two aqueous solutions of the same colloid, one 

 dilute and the other concentrated, to be immiscible with each other. 

 This phenomenon was called coacervation by Bungenberg de Jong 

 and Kruyt^^ because of the 'heaping together' of the constituent 

 micelles in the concentrated phase. A general account of the phenome- 

 non has been given by Dervichian.^' It is greatly influenced by 

 inorganic ions, and the evidence for a coacervate nature of the nu- 

 cleolus rests mainly on the effects upon it of various salts. Hygen 

 observed that the nucleoli of the egg nucleus in Micrasterias fuse under 

 the influence of KNO3, while Duryee reports that a number of reagents 

 will dissolve the nucleoli of the oocyte nucleus of the frog. Temperature 

 is also a factor of importance in coacervation. Ehrenberg^* found that 

 the nucleolus in Salix decreases in volume with increase of temperature 

 and drew the inference that this phenomenon was involved. It may 

 well be that the concept will prove to be widely relevant in biology; but 

 its application to the nucleolus depends on whether this body is in fact 

 a liquid. Very probably, the physical state of the nucleolus varies in 

 different organisms; in vertebrate cells in tissue culture, the nucleoli 

 appear to be solid; when they disappear in such cells treated with 

 distilled water, as Zollinger^^ has described, it is probable that simple 

 solvation is involved. Gersch^^ finds that the solubility in water of the 

 nucleoli of the oocyte nucleus of the frog increases as its development 

 proceeds. 



Nucleoli and chromosomes 



In recent years, morphological interest in the nucleoli has centred 

 mainly round the thesis that they have a definite relationship to par- 

 ticular chromosomes, in association with a special segment of which 

 they are formed in telophase. The development of this view has been 

 traced by Gates. ^^ In plants, the relationship is so well established 

 that, as Gates^° says : 'the number of nucleoli arising in the nuclei at 



D 41 



