I am specifically interested in this within the context of 1950's developmental biology (I presume the meaning and scope of usage of the terms has changed since then), as I am reading Alan Turing's paper on 'The chemical basis of Morphogenesis'. Turing first mentions evocators in the context of 'evocators of Waddington'. I have read some articles by Waddinton to try to see what he meant by 'evocator', but have not gained clarification from the articles available online. Turing cites Waddington's "Organisers and Genes", which is not available on the web and I do not have access to.
A more recent article here states
The first element is the inducer (or evocator) defined as the signal emanating from a piece of tissue, such that when the tissue is transplanted to an ectopic position it elicits changes in the behavior of neighboring cells.13 Today, we understand that most of these inducers are secreted proteins and include morphogens belonging to the Wnt, Hedgehog (Hh), TGFβ, EGF, and FGF families.
Which seems to imply that most evocators are morphogens, although from the definition of evocator given I would say that this is exactly what a morphogen is!
Any insights would be much appreciated!