part of ends during starts during involuting marginal zone involuting marginal zone mesenchymal layer XAO:0000075 Deep layers of the IMZ consisting of prospective migrating mesodermal cells (which will migrate into the interior and form the leading edge of the mesodermal mantle) and prospective converging and extending mesoderm. deep involuting marginal zone xenopus_anatomy tissue NF stage 10 NF stage 12