Genie and Non-genic Parts of the Chromosome 59 



though incomprehensible, nevertheless indicates a unified importance 

 of heterochromatin. 



Specific features of heterochromatin are observed also in path- 

 ological cells (cancer cells), but without any definite rule. The 

 structure of chromocenters is highly variable. Some appear to be 

 structureless deep-staining matter; others, lighter staining groups of 

 granules. Certainly, none are dense spiralizations, as one theory 

 requires. Within the chromosomes, heterochromatic sections may 

 appear only as larger or clumped chromomeres with normal spiral- 

 ization of the chromonema; or spiralization may seem to be absent 

 and the heterochromatic portions show an irregular structure. Some 

 authors insist on a definition based upon spiralization, others, upon 

 DNA metabolism. Some consider all transitions between eu- and 

 heterochromatin possible, and even inteipret the size differences of 

 the chromomeres as a transition to heterochromatization (Pontecorvo, 

 1944; White, 1945; Virkki, 1951). Another opinion is that hetero- 

 chromatin is phylogenetically degenerated euchromatin. These great 

 variations both in resting nuclei and in mitotic chromosomes make it 

 very difficult to give a general description or definition of hetero- 

 chromatin. It cannot even be said that it is ubiquitous, because, in 

 view of its great changeability, it might be present even when not 

 seen; furthermore, structures may be called heterochromatic without 

 any certainty that they are homologous to others that have been 

 similarly labeled. This confusion is made worse by the concept of 

 heterochromatization, meaning the change of euchromatic into hetero- 

 chromatic sections. This is frequently claimed in cytogenetic work in 

 Drosophila (Hannah, 1951; and especially the papers of Belgovsky, 

 1938, 1944, 1946; Prokofyeva-Belgovskaya, 1947). Sometimes it is 

 based upon alleged changes in the stainability of sections of salivary 

 chromosomes; sometimes it is inferred from aberrant genetic behavior; 

 and, I dare say, it is always a vague, slippery subject which cannot be 

 accepted as proved or disproved. Unified theoretical considerations 

 are possible only on the basis of indisputable material. Aberrant and 

 queer data may one day become of the greatest significance. However, 

 if it is not possible today to absorb them into the major body of estab- 

 lished facts, the pragmatic attitude is to note their existence but leave 

 them aside for the time being. (We shall return to the idea of hetero- 

 chromatization when discussing the mottling position effect in the 

 presence of heterochromatic breaks and again when discussing the 

 theories of position effect.) 



