How did Edouard van Beneden show that sperm and eggs have half the usual number of chromosomes?

Edouard van Beneden demonstrated that sperm and egg cells contain half the number of chromosomes found in body cells through his investigations of the intestinal worm Ascaris megalocephala, beginning in 1883.

By studying the fertilization process in this species, van Beneden observed that the union of the male sperm cell and the female egg cell involved the joining of two half-nuclei. He showed that each of these nuclei contained only half the chromosome number present in the organism's somatic (body) cells. When these two halves united, they restored the full, characteristic number of chromosomes for the species.

Furthermore, by examining a subspecies of Ascaris that possessed only two chromosomes in its body cells, he was able to clearly reveal the individuality of single chromosomes. His work established the fundamental principle that the chromosome number remains constant for every body cell of a species and that gametes are haploid, containing only half that total count.1