Untersuchungen über den ersten Entwicklungsvorgänge in den Eiern der Insekten. II. Ueber Spermatogenese und deren Beziehung zur Eientwicklung bei Pyrrhocoris apterus.

In his study of the testicles of the firebug (Pyrrhocoris) Henking noticed that one chromosome did not take part in meiosis. He named this X element because its strange behavior made him unsure whether it was genuinely a chromosome. After it was established that the "X element" was indeed a chromosome it became known as the X chromosome.