COMPLETE REGULATION OF DEVELOPMENT THROUGHOUT METAMORPHOSIS OF SEA-URCHIN EMBRYOS DEVOID OF MACROMERES

The developmental potential of the animal cap (consisting of eight mes omeres) recombined with micromeres or of micromere progeny was examine d in sea urchin embryos. The embryos derived from the animal cap recom bined with a quartet of micromeres or their descendants developed into four-armed plutei. After feeding, the larvae developed into eight-arm ed plutei. The left-right polarity of the larvae, recognized by the lo cation of the echinus rudiment, was essentially normal, regardless of the orientation of animal-vegetal polarity in micromeres combining wit h the animal cap. The larvae had sufficient potential to metamorphose into complete juvenile sea urchins with five-fold radial symmetry. Cel l lineage tracing experiments showed that: (i) macromere progeny were not required for formation of the typical pattern of primary mesenchym e cells derived exclusively from large micromeres; (ii) the progeny of large micromeres did not contribute to cells in the endodermal gut wi th three compartments of normal function; (iii) the presumptive ectode rm had the potential to differentiate into endodermal gut and mesoderm al secondary mesenchyme cells, from which pigment cells likely differe ntiated; and (iv) behavior of the progeny of small micromeres was the same as that in normal embryos through the gastrula stage. These resul ts indicate that the mesomeres respecify their fate under the inductiv e influence of micromeres so perfectly that complete juvenile sea urch ins are produced.