Heterochromatin/Euchromatin?

Heitz (1929) originally described as heterochromatin that portion of the nuclear chromatin which demonstrated its allocycly by maintaining a condensed state throughout cell interphase while the remainder of the nuclear chromatin was extending to what he termed the euchromatin state. In plane english: heterochromatin is an inactive part of chromatin (doesn't participate in transcription) and euchromatin contains most of the transcriptionally active genes. Of course at each stage of development or/and cell differentiation there are different parts of DNA (chromatin) that became activated. In situ hybridization provides and insight on this differentiation.