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portions of the chromosomes that remain thickened in the interval between cell divisions, that is, in the interphase (as opposed to other portions called euchromatin). Heterochromatin is sometimes closely associated with the nucleolus, forming a sort of ring or casing around it. During mitosis the heterochromatin is colored more or less darkly than the euchromatin (the phenomenon of positive or negative heteropycnosis). Heterochromatin is especially characteristic of sex chromosomes of many animal species. Heteropycnotic areas can be obtained experimentally, for example, through the action of low temperature. It is theorized that heterochromatin does not contain genes that control the development of the organism.
M. E. ASPIZ