Abstract

Organismal form arises by the coordinated movement, arrangement, and activity of cells. In metazoans, most morphogenetic programs that establish the recognizable body plan of any given species are initiated during the developmental period, although in many species growth continues throughout life. By comparing the cellular and molecular development of the bilaterians (bilaterally symmetrical animals) to the development of their closest outgroup, the cnidarians, it appears that morphogenesis and the cell fate specification associated with germ layer formation during the process of gastrulation are separately controlled by two distinct downstream pathways (canonical and planar cell polarity) of Wnt signaling. Furthermore, a fundamental change in the early developmental program, the positioning of the site of gastrulation, allowed the spatial separation of gene regulatory networks that facilitated the rapid diversification of bilaterian body plans