CHROMOSOMES AND GENES 



57 



physics, we might say that the theory of 

 the chromosome had reached a stage repre- 

 sented in the history of physics by the 

 theory of the indivisible atom of my youth, 

 before Thompson, Rutherford, Planck, and 

 Bohr. 



After this short introduction, the first 

 problem under the heading of our paper is 

 this : During the development of the classic 

 theory chromosomes were practically iden- 

 tified with chromatin, i.e., nucleic acid, 

 though different structural elements within 

 the chromosome were already known partly 

 since the earliest cytological work ("Pfitz- 

 ner's Korner," chromomeres, chromioles, 

 spiral thread). Boveri, for example, 

 thought of genes always in terms of chro- 

 matin, and Baur used the term chromo- 

 mere as late as 1929 as an equivalent of 

 gene. Our first problem, then, is to discover 

 the relation of the chromatin to the heredi- 

 tary material or genes. To be sure, cytolo- 

 gists working without special application to 

 genetics had long since distinguished be- 

 tween a chromatin of a general function in 

 nuclear and cellular affairs and the he- 

 reditary chromatin, trophochromatin and 

 idiochromatin (Riickert, Lubosch, Gold- 

 schmidt). I am sure that the facts 

 underlying this distinction (especially those 

 taken from oogenesis) will receive new sig- 

 nificance when they shall be reattaeked with 

 present day chemical methods. The same 

 applies to a set of facts of greatest impor- 

 tance which cytogeneticists of today have 

 completely forgotten : namely, chromatin- 

 diminution in nematodes and cecidomyids, 

 the sloughing off of chromatin in the ma- 

 turation divisions of lepidopteran eggs, the 

 unequal distribution of chromatin in the 

 oogonial divisions of Dytiscus, the forma- 

 tion of the odd nuclear vesicles in the eggs 

 of ants, and also the existence of the macro- 

 nucleus in Infusoria. The sheer mention of 

 such facts involves a whole program. 



Returning now to the problem of the 

 chemical constituents of the chromosome, 

 there has been considerable theorizing in 

 face of the fact that exact tools for an attack 

 have become available only recently. But 

 it seems as if some of the views derived 



from theoretical consideration did not fall 

 far off the mark. As long as 25 years ago 

 I pointed out that the always chemically 

 identic nucleic acid must play some general 

 physico-chemical role in regard to the genie 

 substance. My idea was that the chromatin 

 is a kind of ground-substance which is able 

 to adsorb definite numbers of gene mole- 

 cules, thus keeping these constant and 

 simultaneously accounting for the equal 

 division of the gene. Though the details 

 of this hypothesis are certainly wrong, the 

 general trend of ideas has recently taken 

 the same direction (see below, Caspersson 

 and Schultz). It was Koltzoff who first 

 assumed that the chromosome contains a 

 genomatic protein-core surrounded by 

 chromatin or nucleic acid. When Meyer 

 and Mark discovered that fibers are made 

 up of micellar aggregates of long chain 

 molecules, I pointed out that such a struc- 

 ture would also furnish a good model for 

 the genie substance, an idea which soon was 

 taken up also by Koltzoff. This assumption 

 appears again in the first strictly chemical 

 theory of chromosome structure developed 

 by Wrinch, who assumes a core of micellar 

 bundles of different polypeptide chains, 

 attached serially to each other and held to- 

 gether by bonds to the free valencies of 

 nucleic acid. The molecules of thymo- 

 nucleic acid are thus assumed to be ar- 

 ranged, in regard to the core, as the warp 

 in a tissue to the woof. This latter concep- 

 tion has meanwhile turned out to be errone- 

 ous. Signer, Caspersson, and Hammarsten 

 showed that thymo-nucleic acid (chroma- 

 tin) is a highly polymerized compound with 

 long molecular chains. Astbury and Bell 

 found, by X-ray analysis, that long chains 

 with a period similar to that of polypeptids 

 are involved, and W. J. Schmidt found also 

 by polarimetric methods an arrangement 

 parallel to the long axis of the chromosome. 

 There is further the information gained by 

 Caspersson that the nucleic acid is found 

 only in the chromomeres and that it seems 

 to be synthesized there in the prophases of 

 mitosis. The actual chemical relation be- 

 tween the nucleic acids and the protein-core, 

 which latter is assumed to be the actual 



