The present paper is an in-depth study of the lexical units transferred by metaphorical extension from general English to the field of biology, based on the analysis of a 20 million word corpus of scientific articles. Terminological metaphors are analysed both as keys to cognitive processes involved in scientific activity and as linguistic units. The paper examines the role metaphors play in the evolution of the discipline, as well as the process by which they are introduced, reinforced by common usage, and ultimately lexicalized. The metaphorical term candidates extracted from the corpus are analysed as representing different stages in a diachronic process of lexicalization, or demetaphorising. They are classified according to the perceptual basis (similarity of shape, function or position) or to the `root' conceptual metaphors on which they are built.