This paper “maps” a number of trajectories through which the conceptual contours of sex could be traced in the bioscientific discourse of Republican China. Focusing on the writings of the embryologist Zhu Xi (1899–1962), I analyze the epistemic functionality of such biological terms as ci (“biological femaleness”) and xiong (“biological maleness”) that acquired an unprecedented scope of cultural discursiveness in China only alongside the arrival of Western biology, which replaced classical learning and natural studies as the authoritative field of inquiry about life. I first show that when Chinese scientists used these terms to describe the sex of biological species, they relied on an epistemological framework of visual knowledge that granted some foundational operative power to a signifying order in which one could know by seeing the differences between ci and xiong (and, ultimately, sexual differences). These two terms' lexicality and indexicality thus mutually reinforced one another in the production of their semiotic possibilities and epistemo-logicality. I then show that while they adopted ci and xiong as the bioscientific synonyms of the more culturally anthropocentric words such as nü (woman) and nan (man), Chinese biologists also incorporated sophisticated biological theories of sex from Europe and North America, including the theories of “gynandromorphism” and “intersexuality.” The implicit and explicit figurations of hermaphroditism reveal the ways in which at the heart of the entire bioscientific discourse of ci and xiong resides its key conceptual anchor: the human–non-human divide.