Johannes Hjorth and I have just pulished our short commentary on Grimbert and Cang’s 2012 model of the development of retinocollicular development. Our main point  is that there appears to be an inconsistency between some equations written in their Methods and the meaning of the equations as described in the Results. Were the equations are constructed as outlined in the Results, the model probably won’t work as advertised. There’s also a bit of discussion about the meaning of the word “permissive”.