Histology is a child of the 19th century, and its status in medicine was long in doubt. Was "microscopic anatomy" to be viewed as a refinement of the traditional discipline or as a technically complex subject of doubtful practical value? This was one of the questions that faced the commission charged with reforming examinations in German medical schools in 1878. One of its members, Carl Friedrich Flemming [1799-1880], was able to refer this matter to an acknowledged expert, his son Walther Flemming, who was Professor of Anatomy and Histology at the University of Kiel. Walther's views are contained in a letter that was recently discovered among Carl Friedrich's papers held in the State Library of Mecklenburg-Vorpommern in Schwerin. To the best of our knowledge, this letter is the only surviving piece of personal correspondence from the hand of Walther Flemming, one of the most distinguished biologists of his time. The original text, together with an annotated English translation, is published here for the first time. The letter throws new light on Flemming's attitudes to medical education, and also illuminates his relationship with his father, himself a figure of considerable stature in psychiatric medicine in 19th-century Germany. The younger Flemming [1843-1905], an acknowledged master of microscopy, made his most distinguished contributions to the field we now call cell biology. In addition to the terms chromatin and mitosis, we owe to him the first comprehensive account of the latter, as well as the fundamental insight that meiosis involves two successive divisions.