REGULATIVE CAPACITY OF THE ARCHENTERON DURING GASTRULATION IN THE SEA-URCHIN

Gastrulation in the sea urchin involves an extensive rearrangement of cells of the archenteron giving rise to secondary mesenchyme at the ar chenteron tip followed by the foregut, midgut and hindgut. To examine the regulative capacity of this structure, pieces of the archenteron w ere removed or transplanted at different stages of gastrulation. After removal of any or all parts of the archenteron, the remaining veg 1 a nd/or veg 2 tissue regulated to replace the missing parts, Endoderm tr ansplanted to ectopic positions also regulated to that new position in the archenteron. This ability to replace or regulate endoderm did not decline until after full elongation of the archenteron was completed. When replacement occurred, the new gut was smaller relative to the re maining embryo but the recognizable morphology of the archenteron was re-established. Long after the archenteron reveals territorial specifi cation through expression of specific markers, the endodermal cells re main capable of being respecified to other gut regions. Thus, for much of gastrulation, the gut is conditionally specified. We propose that this regulative ability requires extensive and continuous short-range communication between cells of the archenteron in order to reorganize the tissues and position the boundaries of this structure even after e xperimental alterations.