THE NUCLEUS 55 



matic (stainable with acid dyes). In such cases the appearance of the 

 reticulum suggests a framework of one substance supporting differently 

 stained particles of another substance 



The interpretation of the above appearances has long been a major 

 cytological problem, and no acceptable conclusion has yet been reached 

 with regard to many points. The view was long ago put forward that the 

 reticulum comprises two distinct elements: a supporting framework 

 composed of relatively achromatic linin upon or in which are carried 

 ''granules" of a second, highly stainable substance, chromatin. The 

 chromatin granules were held to be composed of " oxychromatin " or 

 "basichromatin" according to their varying ratio of nucleic acid and 

 protein. '^^ Thus the "chromatin" may be highly basichromatic (strongly 

 acid) during nuclear division, but only weakly so or even oxychromatic 

 (basic in reaction) in the metabolic nucleus where it is in a different 

 functional state with its nucleic acid presumably combined with basic 

 substances. At all stages it reacts positively to the Feulgen test (p. 52). 

 Physical factors, such as degree of dispersion, as well as the order in 

 which stains are applied, may also affect the results obtained in 

 preparations.^^ 



The evidence for such a structural distinctness of chromatin and 

 achromatic supporting material in the reticulum has been questioned by 

 many workers. 2° It has been suggested that if there are two elements, 

 chromatin and linin, they are not so distinct morphologically as the 

 earlier workers supposed, the chromatin existing rather as a thin fluid 

 impregnating the linin substratum. The chromatic lumps are often not 

 sharply set off from the rest of the thread but taper off gradually. In 

 such cases it has been found that purposes of cytological description are 

 well served by the conception of a reticulum composed of a single com- 

 plex substance which stains variously in different regions and at different 

 stages of the nuclear cycle according to the size of the strands, their 

 physico-chemical state, and the technical procedure employed. This 

 one substance is loosely spoken of as chromatin, but because of the long 

 application of this term to a supposedly distinct component of the retic- 

 ulum it is advisable to use Lundegardh's (1910) term karyotiji for the 

 reticular substance as a whole. Only future research can decide whether 

 karyotin ("chromatin" in the wide sense) is a true chemical compound 



1* Heidenhain (1890, 1907). As used by many writers, the term "oxychromatin" 

 includes the Hnin, "chromatin" denoting only the "basichromatin." Heidenhain 

 later called the chromatin granules "chromioles," a term introduced by Eisen (1899). 

 See the discussions by Wilson (1925) and Stieve (1921). Oes (1908, 1910) and 

 Nemec (1909, 1910) reported that the two kinds of chromatin differ in solubility. 



" A. Fischer (1899), Hardy (1899). 



2° Gregoire and Wygaerts (1903), Gregoire (1906), Sypkens (1904), Martins- 

 Mano (1904), Lundegkrdh (1910, 1912), Malte (1910), Sharp (1913, 1920a), de 

 Litardiere (19216), McClung (1924). 



