SET-ASIDE CELLS IN MAXIMAL INDIRECT DEVELOPMENT - EVOLUTIONARY AND DEVELOPMENTAL SIGNIFICANCE

In the maximal form of indirect development found in many taxa of mari ne invertebrates, embryonic cell lineages of fixed fate and limited di vision capacity give rise to the larval structures. The adult arises f rom set-aside cells in the larva that are held out from the early embr yonic specification processes, and that retain extensive proliferative capacity. We review the locations and fates of set-aside cells in two protostomes, a lophophorate and a deuterostome. The distinct adult bo dy plans of many phyla develop from homologous set-aside cells within homologous larvae. We argue that the stocks from which these phyla aro se utilized these respective larvae, and the diversity of their adult body plans reflects diverse pattern formation processes executed in th eir set-aside cell populations. Chordates and arthropods develop direc tly, but share adult characters with indirectly developing phyla. Thus the deuterostome and protostome stocks that were ancestral to chordat es and arthropods, respectively, also utilized maximal indirect develo pment.