Prelude to molecularization: The double gradient model of Sulo Toivonen and Lauri Saxén

The present molecular investigations of Organizer phenomena show a remarkable connection to the earlier classical embryological studies that used transplantation as a method for making mechanistic models of induction. One of the most prominent of these connections is the dual gradient model for anterior-posterior and dorsal-ventral polarity. This paper will discuss some of the history of how transplantation experiments provided data that could be interpreted in terms of two gradients of biologically active materials. It will highlight how the attempts to discover the elusive Induktionsstoffen gave rise to the double gradient model of Sulo Toivonen and Lauri Saxén in the 1950s and 1960s. This paper will also document how this research into the identity of these molecules gave rise to the developmental genetics that eventually would find the molecules responsible for primary embryonic induction.