2 NUCLEUS 221 



protein framework. It seems to me that the chemical dualism in the 

 nuclear structure is clearly demonstrated by its optical anisotropy, as 

 the character of the birefringence is determined either by the optically 

 positive proteins or the optically negative nucleic acid inclusions. 



Nucleolus. It has been shown that the nucleoli have their origin in 

 the accumulation of proteins (in particular histones, Caspersson, 

 1940a; Serra and Queiroz-Lopes, 1944), which can be regarded as 

 reserve substances. The reserve proteins (e.g. edestin, excelsin, 

 p. 141) differ from the framework proteins by a lower degree of 

 polymerization and the globular form of their molecules. They may 

 be arranged into molecular crystal lattices capable of .=. welling. It is 

 significant that the nucleoli in the nucleus can be substituted by 

 protein crystalloids (Kuster, 1955 a, p. 155) as is the case with repre- 

 sentatives of the Scrophulariaceae (Gicklhorn, 1932b) and Lenti- 

 bulariaceae. Sometimes nucleoli and crystalloids occur simultaneously 

 in the same nucleus (Zimmermann, 1896). 



The principal difference between the proteins of the reticulum and 

 those of the nucleoli is the greater solubiHty of the latter. In contrast 

 to the very resistant nuclear frame, they are readily dissolved by 

 pepsin in hydrochloric acid. In spite of its high histone content, the 

 nucleolus apparently possesses weak anionic properties, for it vigor- 

 ously collects most basic dyestuffs and as a rule shows a greater re- 

 sistance to swelling in dilute alkaline solutions than the reticulum 

 (TisCHLER, 1921/22, p. 45-51). This is due to a certain content of 

 globuHns and ribonucleic acid (Caspersson, 1941), so that its I.E. P. 

 does not lie in the alkaline field. On the other hand, it also stores acid 

 dyestuffs such as eosin, indicating an I.E. P. near neutrality. The 

 behaviour of the nucleolus towards dyestuffs depends very much on 

 the process of fixing and the method of staining (Romeis, 1943, p. 323), 

 which may modify its isoelectric behaviour. It is well kown that 

 Carnoy fixation (alcohol -j- acetic acid) dissolves a good deal of the 

 nucleolus, so that it appears to be surrounded by a broad areola. It 

 shows a specific affinity for the acid dyestuff methyl green (Colour 

 Index, I St ed., No 684, in German Lichtgriin) which allows of dif- 

 ferential staining in comparison with the red Feulgen reaction of 

 chromatin (Semmens and Bhaduri, 1939). This double staining has 

 become important for the problem of nucleoH formation in the telo- 

 phase of cell division. 



