110 The Structure of Protoplasm 



to avoid drifting again into the relative stagnation from which their 

 subject has fairly recently emerged. The geneticists interested in 

 nuclear structure must know the data of the first and second groups. 

 The third group needs to know all the data of all the groups. 



Much of my own work falls jointly into fields two and four. At 

 present I am particularly impressed, perhaps depressed, by the 

 divergencies of opinion between workers on the second level — the 

 one with which I am most familiar. These I consider the greatest 

 source of danger for those workers of the other groups who seek to 

 bolster or check the hypotheses formed from their own observations 

 by the data of the second group. This difficulty in its relation to vari- 

 ous aspects of the general problems of nuclear structure will there- 

 fore be stressed. 



Studies on the presence and distribution of nucleic acids made 

 jointly on the microscopic and submicroscopic levels, have been of 

 particular interest in recent years. The studies with visible light 

 have depended largely on the Feulgen reaction; those at a lower 

 level on the ultraviolet absorption technique developed by Caspers- 

 son. With the Feulgen method (the application of Schiff's reagent 

 to the aldehyde groups of the pentose constituents of nucleic acids) , 

 desoxyribose (ribodesose-, "thymo-") nucleic acids give a positive 

 staining reaction, while ribose ("yeast-") nucleic acids do not stain. 

 This test is, however, of only limited value in making quantitative 

 determinations of thymonucleic acid in cytological preparations, 

 The method of Caspersson (1936, 1940) permits quantitative deter- 

 minations of nucleic acids, but the ultraviolet absorption spectra 

 with characteristic maxima at 2,600 A, "being determined by the 

 nitrogenous constituents," do not differentiate between the desoxy- 

 ribose and ribose types. For the study of the nucleic acid distribution 

 in the cell, a combination of both methods is thus indicated. 



Studies by the Feulgen method have shown clearly that desoxy- 

 ribonucleic acid is concentrated in the chromosomes and is either 

 absent or present only in very much smaller amounts in the cyto- 

 plasm and nucleolus. Since thymonucleic acid appears to be the 

 "typical chromosome nucleic acid" Caspersson (1939) believes that 

 it is synthesized on the chromatid. The numerous objections to the 

 use of the Feulgen method in cytology have almost all been ade- 

 quately dispelled by Milovidov (1938) , Hillary (1940) , and others 

 who showed that when properly used, "chromatin" and only chroma- 

 tin is stained by it, as Feulgen claimed. The remaining difficulty, 

 that at certain stages in the maturation of the animal egg the chromo- 



