22 Nature of the Genetic Material 



correctly, sets of chromomeres), just as typically different and specific 

 as the others, and that each one synapses with its homologue. The 

 number is roughly a thousand for a large chromosome arm. Clearly, 

 still more despiralizing must be involved. There is do reason to assume 

 that the series has reached its end here. Kodani ( 1947 ) has shown that 

 under certain conditions a number of small bands may contract into a 

 single thicker band. Hence the opposite might be expected, namely, 

 the despiralization of individual bands into still smaller units — in the 

 present discussion, chromomeres of fourth grade. The limits of micro- 

 scopic visibility are reached here, but there is no reason why this sub- 

 division could not continue to the submicroscopic and finally the 

 molecular level. 



At this point some genetic facts must be taken into account. The 

 technique of the salivary gland chromosomes sometimes allows a 

 localization of a definite mutant locus within a single visible band. 

 For example, the absence, deficiency, of a single band produces 

 the mutant effect or the typical effect of a deficiency for the locus in 

 question. Some optimistic geneticists do not hesitate to identify the 

 bands with the genes of classic genetics. We shall later see why this is 

 not possible. Here I shall mention only two facts: first, the mutant 

 effect can be produced also by a rearrangement break between two 

 chromomeres ( position effect ) ; second, it can be produced, too, by the 

 complete absence of a band, as in homozygous yellow deficiency. The 

 fact remains that the chromomeres must be of some importance in 

 the genie function of the chromosome, even if they are not the genes 

 themselves. A newer and more helpful theory of the genie material as 

 related to the morphology of the chromosome will be presented later. 



The chromomeres contain the major part of DNA. In the lamp- 

 brush chromosomes of the vertebrate oocyte, which may have a 

 diameter of only about 150-200 A (reports vary from 150 to 1,000) 

 and may thus be single molecular chains, Feulgen positive chro- 

 momeres are present in definite intervals, and the loops are attached 

 to them. Nevertheless, between the chromomeres the chromonema 

 probably also contains DNA, but so scattered that the Feulgen re- 

 action does not always show it. These chromosomes certainly look 

 very different from those of cleavage cells or tissue cells, but the 

 constancy of the DNA content holds for these nuclei as for all others. 

 It is very difficult to imagine this, if the variable visible structure 

 indicates a really different charge or incrustation with DNA locally. 

 It is expected, instead, that DNA is present along the entire chro- 



