Fig. 1. Experimental procedures. (A) Mesoderm migration on conditioned substratum. (Left) Mesoderm cells migrate on the extracellular matrix (ECM) of the BCR towards the animal pole (AP) (arrow). Blue, prospective ectoderm; pink, mesoderm (M); yellow, endoderm (E). (Right, top) A strip of BCR (bracket in left figure) is explanted at stage 10, and cultured for 1 hour with its matrix-forming surface down. (Right, bottom) The ECM of the BCR has become deposited at the bottom of the dish. A mesodermal explant (M) placed on the conditioned substratum migrates (arrow) to the animal pole position. (B) Construction of trPDGFA. The sequence corresponding to amino acid residues 198-227, that contain the matrix binding motif, was deleted from the lfPDGFA.