Since the turn of the twentieth century, chromosomes prepared on microscope slides formed clumps that made it extremely difficult to distinguish them. Although the preparations made the identification of individual chromosomes difficult, by the 1920s cytologists consistently reported a diploid number of 48 human chromosomes. In April 1952, Hsu discovered a technique—the hypotonic solution—that separated the clumped chromosomes, allowing him to observe each one individually. Even though he now could distinguish human chromosomes to a much greater degree than his predecessors, Hsu still reported a diploid number of 48 human chromosomes (see Figure 14 in his 1952 paper). The correct diploid chromosome number of 46 human chromosomes was first reported three years later by Joe Hin Tjio and Albert Levan.

See Hsu, T. C. Human and mammalian cytogenetics: An historical perspective (1979). See also Kottler, Malcolm Jay, "From 48 to 46: Cytological technique, preconception, and the counting of human chromosomes," Bull. Hist. Med., 48 (1974) 465-502.