Chromosomes

Chromosomes became visible only when microscopes met chemistry. Walther Flemming, working at the University of Kiel in the early 1880s, discovered that certain aniline dyes stained thread-like structures in dividing cells. He called the staining substance chromatin—from the Greek for color—and named the division process mitosis, from the Greek for thread.

The adjacent possible had converged from three directions. German optical engineering—Zeiss and Abbe in Jena—had produced microscopes capable of resolving cellular detail. The synthetic dye industry had created aniline stains that selectively bound to nuclear material. And cell theory, established by Schleiden and Schwann four decades earlier, had focused biological attention on the cell as life's fundamental unit.

Flemming observed something extraordinary: during cell division, the chromatin condensed into distinct rod-shaped bodies that split lengthwise, with half going to each daughter cell. He published his findings in Cell Substance, Nucleus, and Cell Division (1882), a work that established cytology as a discipline.

The term chromosome came later—Heinrich Wilhelm Waldeyer coined it in 1888, combining Greek words for color and body. By then, other researchers had confirmed Flemming's observations across species. Every dividing cell showed the same choreography: chromatin condensing into chromosomes, chromosomes aligning and splitting, daughter cells receiving identical complements.

The significance for heredity remained unclear until 1902, when Walter Sutton and Theodor Boveri independently proposed that chromosomes carried Mendel's hereditary factors. The chromosome theory of inheritance unified Flemming's microscopy with Mendel's mathematics—genes resided on chromosomes, and chromosome behavior during cell division explained inheritance patterns.

Flemming had seen the physical substrate of heredity without knowing what he was looking at. The colored threads he stained contained the instructions for building organisms. Aniline dyes had revealed what evolution had encoded over billions of years.