186 BO THORELL 



usually after treatment with histological fixatives since such treatment in 

 some special cases can cause a redistribution of DNA within the cell.-^ 



In suitable material some details of localization of DNA in the chromo- 

 somes can be studied by the Feulgen reaction. The giant chromosomes in 

 salivary gland cells of Drosophila are regarded as bundles of extended, 

 similar chromonemata lying side by side.-^"*** Each chromonema carries 

 Feulgen-positive granules of various sizes. The corresponding granules of 

 adjacent chromonemata are arranged across the width of the chromosomes 

 in the form of bands. These granules have been claimed to represent the 

 gene loci.^''^^ 



The threadlike structures isolated from interphase nuclei and considered 

 to be chromosomes (see Sect. II. 1) also show a positive Feulgen reaction'' 

 with, in many cases, beadlike formations along paired filaments.*'^ The 

 different pictures of the various types of interphase nuclei after staining 

 with the Feulgen nucleal reagent can be explained by the different mode of 

 packing of the chromosome threads. Furthermore, in many cell types, the 

 Feulgen-positive material (DNA) of the chromosomes tends to coalesce 

 forming a chromocentrum, the functional significance of which is unknown. 



During mitosis the chromosomes can usually be beautifully demonstrated 

 in situ by staining the DNA with the Feulgen procedure.^^''^ The struc- 

 tural changes mentioned in the introduction can thus be followed in the 

 essential details (see also Sect. III). 



h. Staining Methods 



Investigations of nuclear substances with acid and basic dyes date from 

 the work of Fleming (1875-81),^^ who defined chromatin as a substance 

 showing strong affinity for dyestuffs and stated: "Es ist moglich, dass diese 

 Substanz geradezu identisch ist mit den Nukleinkorpern. . . ."It was shown 

 by several workers that staining was dependent more or less on the charge 

 on the cellular substances. Malfatti (1892)*^ used mixtures of acid fuchsin 

 and the basic dye methyl green. With nucleic acid and protein compounds 



" J. Chayen and K. P. Norris, Nature 171, 472 (1953). 

 28 E. Heitz and H. Bauer, Z. Zellforsch. 17, 67 (1933). 

 ^^F,. Heitz, Z. rndukt. Abstamm.-u. Vererblehre 67, 216 (1934); see also H. Bauer, 



Zool. Jahrb. Physiol. 56, 239 (1936). 

 30 T. S. Painter, J. Hereditn 25, 465 (1934). 

 "T. S. Painter, Genetics 19, 175 (1934). 

 52 C. B. Bridges, Am. Naturalist 56, 51 (1922) ; see also T. H. Morgan, C. B. Bridges, 



and J. Schultz, Carnegie Inst. Wash. Yearbook 33, 274 (1934). 

 " H. Voss, Z. Mikroskop-anat. Forsch. 33, 222 (1933). 

 " A. Hughes, "The Mitotic Cycle." Butterworth, London, Academic Press, New 



York, 1952. 

 '* W. Flemming, "Zellsubstanz, Kern und Zellteilung." Leipzig, 1882. 

 "H. Malfatti, Z. physiol. Chcm. 16, 68 (1892). 



