thickening describes the morphogenic movement seen in the marginal zone
of sandwich explants, in which cells converge along the mediolateral
axis while thickening in the radial axis of the embryo. In
explants of ventral marginal zone, this process operates beginning in
early gastrula stages and continuing into early neurula stages.
In explants that include the dorsal marginal zone (the organizer),
convergent thickening begins as in