 However, extrapolation from bacterial gene regulation to development in multicellular organisms proved unfounded. Limitations of the operon model soon became evident. In eukaryotic, multicellular organisms, gene expression is far more complex. Fifty years of molecular biology have revealed a menagerie of mechanisms implicated in the complex cellular machinery of gene expression (see the entry on  molecular biology).  And this is not to mention interactions among cells and tissues, about which classic molecular biology was silent. Brian Goodwin, a student of Waddington’s, charted an alternative theoretical path that skirted these limitations. In the 1960s, he developed equations to model genetic oscillation, building on Jacob and Monod’s operon model (Goodwin 1965). But his subsequent theorizing about development emphasized self-organization due to chemical gradients and mechanical strain in cytoplasm, presaging dynamic and physical theories that gained prominence around the turn of the millennium (Goodwin &amp; Trainor 1985, see  Section 10.2  and the entry on  systems and synthetic biology).