2 NUCLEUS 219 



dividing. A more convincing proof is, however, brought forward by 

 Caspersson's experiments (1936), which are based on the absorption 

 of ultraviolet hght bv nucleic acids. By means of microphotometric 

 measurements he shows that the concentration of nucleic acids in the 

 nucleus strongly increases in the preliminary stage of cell division, to 

 decrease again during the telophase. 



It is perhaps partly due to changes in nucleic acid content that fixed nuclei 

 are sometimes more easily stained with acid dyestuffs (erythrophily), at 

 other times with basic ones (cyahophily), as has been summarized by 

 TisCHLER (1921/22). Caspersson, howcvcr, has not been able to establish 

 a relation between nucleic acid content and basic or acidic nuclear reaction 

 with respect to dyestuffs in the nuclei of the gland of the oesophagus of 

 Helix pomafia. This must probably be explained by the fact that not only the 

 number of acidic or basic groups in the nuclear proteins, but also the pn 

 of the karyolymph is determinative for the anionic or cationic behaviour 

 of the nucleus (Keller, 1932; Becker, 1936). On the other hand, the 

 nucleic acid content probably determines the I. E. P., so that at a constant 

 pfj value the adsorptive power of a nucleus towards basic or acid dyestuffs 

 may vary. 



Caspersson's photometric determination of nucleic acid seems to 

 prove that the increasing chromophily of the fixed nucleus is related 

 to the accumulation of nucleic acids in the preliminary stages of cell 

 division. It seems to me that the older cytologists, who distinguished 

 in the nuclear substance a component Hke plastin, linin or achromatin 

 (difficult to stain and hardly digestible) from the easily stained "chro- 

 matin", already recognized the existence of the two fundamental 

 principles in the nuclear structure viz., on the one hand high poly- 

 meric, relativelv resistant proteins and, on the other, a compound very 

 sensitive to basic staining, the nucleic acid, which predominates during 

 nuclear division but falls into the background in the active nucleus. 

 The well-known staining of fixed nuclei with basic dyestuffs indicates 

 the presence of liberated acid groups and the nucleal reaction points 

 to aldehyde groups. Undoubtedly therefore, the chromatic substance 

 consists mainly of nucleic acids. In spite of this it is not possible to 

 designate these chemically well-defined compounds as chromatin. For, 

 in cytology, the term chromatin has become a morphological concept 

 for regions showing identical behaviour with respect to staining 

 (Heitz, 1935). Those regions of the active nucleus or parts of chro- 

 mosomes which after division do not lose their high nucleic acid 



