220 FINE-STRUCTURE OF PROTOPLASM II 



content, are designated as heterochromatic (positively heteropycnotic, 

 White, 1945). In other words, heterochromatin comprises those thymo- 

 nucleic acids which remain passive during the changing phases of 

 mitosis, whereas chromatin or euchromatin consists of thymonucleic 

 acids which in the process of nuclear division first increase and after- 

 wards decrease again. It has been found that the heterochromatic re- 

 gions (chromocentres) of a nucleus represent chromosome parts which 

 locally have preserved their spiral structure (Straub, 1943). 



Birefringence of the nucleus. As a rule spherical nuclei are isotropic 

 aside from their birefringent boundary. If their shape is anisodia- 

 metric, however, they often display double refraction. 



The two components, protein and nucleic acid chains, do not only 

 show opposite chemical behaviour, in that the one is positive (cationic) 

 and the other negative (anionic). Their optical reactions are also op- 

 posite. In the natural state all fibrillar proteins investigated so far are 

 optically positive, whereas, according to the interesting model experi- 

 ments of Schmidt (1957a) and the experiments on flow birefringence 

 (Signer, Caspersson and Hammarsten, 1938; Wissler, 1940), arti- 

 ficially prepared fibres of the sodium salt of a-thymonucleic acid are 

 optically negative. For this reason elongate nuclei with a high nucleic 

 acid content, Hke certain sperm nuclei (Fig. 125a, p. 228), are optically 

 negative. The negative reaction in polarized light is, however, limited 

 to the chromatic part of the sperm head. Often the achromatic parts 

 are optically positive. Any attempt to explain this positive reaction as 

 rodlet birefringence, i.e., as positive textural double refraction, is 

 inconclusive in the absence of indisputable Wiener curves. Since 

 Schmidt (1937a, p. 87) has proved that these regions show positive 

 intrinsic birefringence, it seems to me that the anisotropy must be 

 attributed to the submicroscopic protein framework. I do not doubt 

 that it exists also in the chromatic part of the sperm head, where it 

 is over-compensated, however, by the strongly negative nucleic acid. 

 If it were possible to eHminate the nucleic acid components com- 

 pletely without disturbing the structure and to dehydrate the protein 

 to a sufficient extent, both the positive rodlet birefringence and the 

 positive intrinsic double refraction of the protein framework would 

 become apparent. The positive birefringence of achromatic oblong 

 nuclei, such as the fibrous spindle-shaped nucleus of Aloe described 

 by Kuster (1934b), must doubtless be attributed tothe orientated 



