2 NUCLEUS 217 



uncoiled chromosomes, the question arises whether they are em- 

 bedded in the karyolymph as free corpuscular dispersed particles, or 

 whether they occupy definite relative positions forming a structure. 

 I am convinced that the latter is true, for in general the nucleolus 

 remains in contact with the chromosome fibrils, on to which it has 

 been condensed (Heitz, 193 i; Geitler, 1940), and heterochromatin 

 (cf. p. 220), if present, occupies a certain position in the nucleus and 

 cannot be readily displaced. 



The karyolytnph (enchylema), on the contrary, appears to be a sol. 

 In Allium nuclei, for instance, Luyet and Ernst (1934a) succeeded 

 in separating it from the framework substance of greater specific 

 weight by centrifugal means. The nuclear sap of oocytes of Xenopus 

 laevis is a solution of proteins; its hydrolysate yields a paper chromato- 

 gram with 12 amino acids but no nucleic acid (Brown, Callan and 

 Leaf, 1950). 



The nuclear membrane varies greatly in thickness. According to 

 Luyet and Ernst (1934b) it is not a self-consistent skin, but only a 

 phase boundary. Other authors, however, mention a real envelope, the 

 birefringence of which has frequently been found to differ from that 

 of the nucleus itself. Schmidt (1939c) gives evidence of an optically 

 negative spherite texture in the boundary layer of the nucleus (lamellar 

 birefringence caused by protein chains running in a tangential di- 

 rection). According to F. O. Schmitt (1938) the sign of the spherite 

 cross is reversed after imbibing with glycerol, urea or sugar solutions ; 

 this would neutralize the form birefringence, and the intrinsic bire- 

 fringence of the lipids would become apparent. Pfeiffer (1944) has 

 even published complete curves of form birefringence. Monne 

 (1942 c) believes- the nuclear envelope to be a double membrane, con- 

 sisting of a firm nuclear protein layer free of lipids and a very tender 

 cytoplasmic protein-lipid membrane. The same conditions are de- 

 scribed by Baud (1949 a, b) for the nucleus of liver cells. He empha- 

 sizes that in living nuclei there is no optical anisotropy; only after 

 fixation does a birefringent nuclear membrane appear which is sur- 

 rounded by a birefringent perinuclear zone. The optical anisotropy 

 of the nuclear membrane is that of a negative spherite indicating a 

 protein lamellar texture, whilst the perinuclear layer represents a 

 positive spherite due to radially oriented lipid accumulation around 

 the nucleus (cf. Fig. 118, p. 206). 



