Structural Diferentiation of the Nucleus 111 



somes cannot ordinarily be stained by Feulgen, has recently been 

 removed by Brachet (1940) , who finds, contrary to Koltzoff (1938) 

 and others, that the chromosomes are never entirely Feulgen- 

 negative during oogenesis. It is easy to see them throughout all 

 meiotic stages in Triton, and also in many insects, but they tend to 

 disappear at mid-prophase in the frog. Brachet concludes that it is 

 the changing size of the chromosomes and the degree of dispersion 

 of the thymonucleic acid that causes variation in staining capacity. 

 Here is the first of several examples where differences of opinion 

 between workers on the microscopic level can lead to extreme 

 divergence in conclusions about the submicroscopic. Koltzoff, find- 

 ing no reaction with Feulgen at certain stages, says that thymo- 

 nucleic acid cannot be an essential component of the gene, but only, 

 perhaps, its protector. Brachet, finding some Feulgen positive 

 particles at all stages, cannot agree. 



With his ultraviolet absorption technique Caspersson (1936) 

 showed that: during mitosis nucleic acid is localized in and around 

 the chromosomes, and in collaboration with Hammersten and others, 

 he supplemented the absorption determinations with digestion 

 experiments in which trypsin preparations containing lanthanum 

 dissolve the protein and leave the nucleic acid as insoluble lanthanum 

 thymonucleate. With these two methods it was found that metaphase 

 chromosomes contain nucleic acids and proteins intimately mixed — 

 not as protein islands with surrounding zones rich in nucleic acids — 

 and that salivary gland chromosomes comprise segments alternately 

 rich in and free of nucleic acid. The former are the "bands" so 

 clearly seen after aceto-carmine fixation. Their highly organized 

 structure found after trypsin digestion, indicates that they are more 

 likely to be the seat of the genes than the interband regions which 

 were completely digested. 



Mazia and Jaegar (1938) confirmed the trypsin results and per- 

 formed the complementary experiment of digestion with pepsin. 

 This, they found, "did not dissolve any constituent concerned in 

 maintaining the integrity of the chromosome." After treatment with 

 spleen nuclease, the chromosomes could not be stained with either 

 aceto-carmine or by the Feulgen method. They had not, however, 

 been disintegrated by the nuclease, as the ninhydrin reaction for 

 determining proteins stained them a deep blue while the cytoplasm 

 stained a lighter blue. These authors conclude that "the chromo- 

 some must have a continuous protein structure and the nucleic 

 acid must be attached in such a way that it may be split and the 



