Notochord also acts as a positive inducer. Somitic and lateral plate mesoderm however, initially stimulate, then inhibit, and the neural tube has a small inhibitory role throughout. The largest inhibitor is, however, the neural crest. What we have described as induction, and perhaps seen as a simple switching on, is in fact a complex sequence of interacting processes which may switch the induction off as well as on. On a larger scale - Oral and buccal development in the newt.
Lets now try to see how whole regions of embryos, rather than individual organ systems are formed.
Teeth will not form unless jaws are present. We can see immediately the importance of building the right things in the right order in an embryo, but it is just as important to see that this sort of constraint, the dependence of one system on another is what limits evolutionary possibilities. We can't have three eyes, or a nose on top of our head because these changes would influence other systems to an extent that would make the whole process of development untenable. Once we can begin to understand the way in which regions like the buccal region develop we ought to be able to look at rather bigger bits, like the limbs. We don't need to go into great detail, but the limb is a good example of morphogenesis, depending as it does on interactions between an epidermis and underlying mesoderm.