THE INTERPHASE NUCLEUS 



thesis of the Stockholm school that the site of protein synthesis of the 

 whole cell is the nucleolus and its associated heterochromatin, for 

 ScHULTZ^^* in speaking of this genetical change in nucleolar composi- 

 tion, says 'this is the cardinal fact which lies at the base of the theory 

 that the heterochromatic regions have to do with cytoplasmic synthesis 

 in general, by way of precursors deriving from the nucleoli'. How far 

 this theory can generally be applied to all types of cell depends among 

 other considerations on whether the nucleoli are invariably associated 

 with heterochromatic regions of chromosomes. In plants, the satellite 

 threads connecting nucleoli and chromosomes are often Feulgen- 

 negative (Resende^^"). Indeed the term SAT-thread (Sine Acido 

 thymonucleinico) was first used by Heitz^^^ to indicate the absence of 

 DNA therein. 



Baker and Gallan^^^ have suggested that the term 'heterochroma- 

 tin' should be abandoned because such a noun suggests a chemical 

 entity. This argument applies equally to 'chromatin' itself which still 

 serves an indispensable purpose as a general term in Flemming's ori- 

 ginal sense (p2). However, it is possible that if the use of the word 

 'heterochromatin' were relinquished, there might be some decrease in 

 the tendency to generalize from the salivary nucleus of Drosophila to 

 all other cells. 



proteins of THE NUCLEUS 



It has long been known that in the nucleus, proteins of small molecular 

 weight with basic properties are associated with nucleic acids. 

 MiESCHER^^^ isolated an organic base from the heads of salmon sperm 

 which he termed protamine, the molecular weight of which is now 

 known to be intermediate between those of polypeptides and proteins ; 

 ten years later, Kossel^^* found a basic protein of more complex 

 structure in bird erythrocytes which he termed histone. The evidence 

 for the existence of basic proteins in the nucleus still depends mainly 

 on bulk chemical analysis, for cytochemical observations on this 

 subject do not point to conclusions of the same clarity. Some of the 

 difficulties which beset the identification by ultraviolet microspectome- 

 try of proteins in the presence of nucleic acids have been discussed in 

 previous pages (p 12). A cytochemical method for the recognition of 

 arginine (Serra^^^) has been claimed by this authorise to demonstrate 

 the presence of basic proteins in the nucleus. All proteins, however, 

 contain arginine, and although the proportions of this amino acid are 

 high in protamines and histones, a positive reaction for arginine still 

 might be given by a non-basic protein present in large amounts. 

 Results of great interest and possible significance on differences between 

 the histones of nuclei in different tissues in the same animal are begin- 

 ning to emerge in the work of Stedman and Stedman.^^' These authors 



47 



