This is the same morphed sequence that we examined in the section on "Keller sandwiches" in the gastrulation section of this tutorial, but now the focus will be on the later stages of morphogenesis in sandwiches as it relates to neurulation in Xenopus. Note that the tissue adjacent to the dorsal involuting marginal zone (shown in red) here) undergoes convergence and extension to become extremely long and narrow. The tissue that does this is the dorsal non-involuting marginal zone (DNIMZ). This seems to be analagous to the elongation of the "notoplate" described by Jacobson and Gordon in urodeles. The convergence and extension behavior appears to be predicated upon planar inductive signals that originate in the dorsal involuting marginal zone. and are transmitted to adjacent tissue. Such signal transmission followed by a response from the tissue receiving the signal, is an example of the classical phenomenon known as induction. In this case, the induction of convergence and extension behavior is one of the events associated with neural induction, also known as primary embryonic induction, a term coined by Hans Spemann.
