Chromatin is the genetic material of the nucleus consisting of deoxyribonucleoprotein that occurs in two forms during the phase between mitotic divisions: as heterochromatin (seen as condensed, readily stainable clumps) and as euchromatin (dispersed, lightly staining or nonstaining material). During mitotic division the chromatin condenses into chromosomes.
The German biologist Walther Flemming discovered chromatin in 1879 while researching cell division. In his 1879 publication he notes the presence of thread-like substances distributed throughout the cell nucleus and which readily absorbs dyes such as aniline and haematoxylin. Here is an electron micrograph of a rat’s liver cell nucleus (“N is the nucleolus) with its chromatin visible as dark granular clumps:
- https://zenodo.org/record/1591470#.XzE87i26nmo, pages 157-158.