Nettie Stevens

Nettie Stevens used exacting cytological experiments to help establish the chromosomal basis of biological sex. After teaching for several years, she studied at Stanford and Bryn Mawr College, earning her doctorate in 1903 under Thomas Hunt Morgan. Her careful microscopy compared germ cells and chromosomes across several insect species. Studies in Spermatogenesis presented this research in two parts issued by the Carnegie Institution of Washington in 1905 and 1906. Working with mealworms, Stevens showed that sperm cells carried different chromosome complements and connected those differences with male and female development. Her interpretation distinguished the paired X chromosomes from the unequal X and Y combination with unusual clarity. Studies in Spermatogenesis supplied direct cellular evidence at a formative moment in genetics, when chromosome theory was still being tested. Stevens continued research at Bryn Mawr until her death in 1912. Although her scientific career was brief, the precision of her observations secured her place in the history of cytology and heredity.