Masui Kiyoshi (1887–1981), a prominent Japanese geneticist, is best known for inventing the sex-sorting method of chicks and his contributions to experimental genetics in Japan. Masui drew inspiration from Goldschmidt’s sex determination theory and used chickens, transplantation techniques, and his own “chick sexing” methods in his scientific work. This paper examines the intersection of genetics and industrial breeding by tracing the evolution of Masui’s experimental systems. During the early 20th century, poultry farming emerged as a significant industry in Japan, resulting (...) in the development of standardized organisms and techniques for chicken farming. Masui, a professor at Tokyo Imperial University, collaborated with the Imperial Zootechnical Experimental Station to use these organisms as models for sex determination theory while exploring their further industrial possibilities. First, the paper show how Masui viewed chickens as epistemological objects and transformed his anatomical discoveries into standardized industrial practices. Next, it describes how Masui’s collaboration with German geneticist Richard Goldschmidt led to new academic questions about sex determination mechanisms and how he integrated his knowledge of chicken physiology into his research on “experimental gynandromorphs” to elaborate the theories. Lastly, the paper discusses the biotechnological ideals that Masui aimed to achieve and how they were co-constructed with his mass-production method of intersex chickens from the early 1930s. The trajectory of Masui’s experimental systems highlights the dynamic relationship between agroindustry and genetics in the early twentieth century and demonstrates the ‘biology of history’ in which the biological processes of organisms intertwine with their epistemological history. (shrink)