Emil Heitz and the concept of heterochromatin: longitudinal chromosome differentiation was recognized fifty years ago.

The work of Emil Heitz (1892--1965) laid one of the keystones of cytogenetics. Using a new in situ method, he established between 1928 and 1935 the longitudinal differentiation of chromosomes in euchromatin (genetically active) and heterochromatin (genetically inert). He recognized the association of satellited chromosomes with the formation of the nucleolus, co-discovered the giant salivary chromosomes of diptera, and arrived at a cytological and genetic concept of chromosome structure that has been found essentially correct to date. Yet, Emil Heitz did not gain due recognition by his contemporaries, suffered from the political disturbances of his time, and spent almost a lifetime in isolation, bolstered only by the conviction that his scientific work was significant.