April 21, 1843 (a Friday)

Walther Flemming

On this date, the German biologist Walther Flemming was born. By using aniline dyes, he was able to find a structure which strongly absorbed basophilic dyes, which he named chromatin. He reported that chromatin was correlated to threadlike structures in the cell nucleus. The Belgian scientist Edouard Van Beneden (1846-1910) had independently observed them, too, and they were later named the chromosomes (meaning “colored body”) by German anatomist Wilhelm von Waldeyer-Hartz (1836-1921).

Illustration of the book *Zell-substanz, Kern und Zelltheilung* (1882) by Walther Flemming

Flemming’s greatest accomplishment was first describing mitosis in animal cells (1879), one of the major discoveries in the history of science. As one of the first cytologists, Flemming investigated the process of cell division and the distribution of chromosomes to the daughter nuclei, a process he named mitosis from the Greek word for thread. Dividing cells had been observed almost forty years earlier by Carl Nageli, but he misinterpreted evidence of mitosis as something abnormal in the dead cells he had observed. Flemming observed cell division in salamander embryos, where cells divide at fixed intervals, and of course the staining technique he had developed allowed him to observe the chromosomes clearly. The Polish-German botanist Eduard Strasburger (1844-1912) independently identified a similar process of mitosis in plant cells.

Ultimately, Flemming described the whole process of mitosis, from chromosome doubling to their equal partitioning into the two resulting cells, in a book published in 1882. His terms, like prophase, metaphase, and anaphase, are still used to describe the steps of mitosis. His work helped form the basis of the chromosomal theory of inheritance.

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