The chromosome field

Exposure of cells to temperatures between 1 and 4 C leads to a chromosome phenotype characterized by a reduced chromosome diameter and weaker Feulgen staining. The distribution of these “cold‐induced regions” (CIR) is described for 12 species of Trillium. 4 species of Fritillaria and in the amphibian Ambystoma. The distribution is non‐random and, moreover, it shows several regular features. The CIR have their optimal zone of distribution in the proximal regions of the arms, usually starting at some distance from the kinetochore. There is a general increase of the CIR as the arm‐length increases. In a permitted zone which extends to the median regions of the arms, the CIR occur with less frequency. There are 3 blocked zones where they seem to appear only with difficulty or do not show up at all. The chromosome regions studied so far in connection with the chromosome field are the nucleolar organizers, knobs and CIR. Each one has a distinct chromosome phenotype and a regular and characteristic distribution within the chromosome arm frame. Moreover, each of these regions tends to occupy a separate position within the arm frame. It is postulated that genes with similar functions tend to occupy, within narrow limits, corresponding locations within the eukaryotic chromosome, irrespective of its phylogenetic position. A distinction is made between primary and secondary chromosomes within a given chromosome complement.