Fig. 1. Events during mesoderm spreading. (A,B) The events that occur during the dispersal of the mesoderm. (A) Following invagination, the mesoderm flattens down onto the surface of the ectoderm (arrows). The first cells make contact with the ectoderm symmetrically on either side of the midline (arrowheads) and accumulate high levels of the activated form of MAPK (cells marked in blue). (B) The mesodermal cells subsequently undergo an epithelial to mesenchymal transition, divide and spread out away from the site of invagination to cover the surface of the ectoderm (arrows). A gradient of activated MAPK is observed in the mesoderm with the highest levels at the edge of the mesodermal sheet (cells shaded from light to dark blue). (C) A drawing of a cross section through an embryo at an early stage of mesodermal spreading to illustrate the regions shown in D and E (red boxes). (D,E) Electron micrographs of mesodermal cells (m) showing cytoplasmic extensions (arrowheads) protuding towards the ectoderm (e).