220 FINE-STRUCTURE OF PROTOPLASM II 



in a ground mass (matrix) which shows strong nucleal staining. 



It must be mentioned that a spiral structure has been observed onl)^ 

 in large chromosomes. According to Japanese and American cyto- 

 logists (Straub, 1938; HusKiNS, 1 94 1, 1942) the visible helix some- 

 times possesses a spiral structure of its own, in which case the chro- 

 mosome would possess the structure of a doubly wound helix with a 

 primary and a secondary spiral (large and small spiralling). With the 

 aid of the phase contrast microscope Ruch (1945) has shown that in 

 the case of the much-investigated chromosomes of Tradescantia the 

 chromomeres occurring in pairs on the spirally wound chromonema 

 fibrils suggest the existence of the small spiralling; but by judicious 

 considerations of the focal depth of the microscope objectives used, 

 he proves clearly that no doubly wound chromosomes exist (Ruch, 

 1949). The question as to how the helical chromonemata are separated 

 from each other during mitosis without uncoiling is a problem in 

 itself (Matthey, 1941). 



The chromonema theory has gained general importance by the 

 discovery of the giant double chromosomes of the nuclei from the 

 salivary glands of the Diptera. In these marvellous cytological objects 

 homologous chromosomes are united into astonishingly broad and 

 remarkably long ribbons. These gigantic chromosomes may be re- 

 garded as bundles of numerous expanded chromonemata, formed by 

 endomitosis (Heitz, 1935). They are united into strings of consider- 

 able dimensions ; the chromomeres form transversal discs which, by 

 means of staining or the nucleal method, are made visible as numerous 

 crosshnes (Fig. 124). 



The non-stainable, anucleal regions of the chromonemata bundles 

 represent the protein components of the chromosome. It may be 

 concluded that the protein thread is not restricted to the colourless 

 segments, but runs invisibly through the whole chromosome. In the 

 chromomeres the nucleic acid components are localized, thus masking 

 the protein ground mass. Their localization is demonstrated by the 

 nucleal reaction, the ultraviolet absorption and the X-ray absorption 

 method of Engstrom which proves that the Feulgen positive bands 

 contain 2 to 10 times more mass than the Feulgen negative ones.^ 

 Moreover, the ultraviolet microscope with its higher resolving power 

 furnishes proof of the existence of the protein ground mass in the 



^ Engstrom and Ruch (1951). 



