The concept of the morphogenetic field has been used extensively in developmental biology. However, little is known about the mechanisms that partition these broad areas of tissue into the smaller areas which actually form the corresponding structures, and the remaining tissue. In the Mexican axolotl, the heart field forms as the anterior lateral plate mesoderm migrates over the underlying pharyngeal endoderm between stages 14 and 28. We have previously shown that both the mid-ventral and lateral walls of the pharyngeal cavity have considerable inductive capacity at stage 14. If this inductive capability, and the competence of the mesoderm to respond, is retained between stages 14 and 28, a much broader area of mesoderm would be induced than actually participates in heart development. In this paper, we use explant cultures to establish that pharyngeal endoderm retains its inductive activity, and that both pre-cardiac mesoderm and lateral plate mesoderm caudal to the pharyngeal cavity remain competent to respond to the induction throughout this period. We also map the specified region of the antero-lateral mesoderm between stages 14 and 28 by placing carefully measured areas of mesoderm in culture without inductive endoderm. We found that the region capable of initiating a spontaneous beat approximately doubles in size during this period. Since the specified region is larger than the actual heart primordium, some mechanism must exist to partition "induced" mesoderm into heart-forming and non-heart-forming areas.(ABSTRACT TRUNCATED AT 250 WORDS)