Robert W. Old may be overestimating the difficulties students may have with the book Principles of Development by me and others (Nature 392, 325; 1998).
It is to be hoped that they will not read single sentences in isolation from the rest of the text. Just four lines up from the offending sentence referring to Toll receptor activation, we describe Spaetzle binding to Toll. Indeed, on this and on the previous page, and in several figures, it is made clear that Spaetzle is the ligand for the Toll receptor. Students may be more intelligent than Old supposes.