(A) During the 16-cell stage, a cluster of four small cells called the micromeres (red) form at one pole of the embryo. (B) The micromeres then develop into primary mesenchyme cells (also shown in red) and migrate into the embryo. (C) As the embryo turns into a hollow ball of cells, the primary mesenchyme assembles into a ring with two clusters of cells (red), positioned ventrally. During this phase, the cells fuse and begin to secrete a calcium carbonate skeleton (shown in blue). (D) In the larva, the skeleton has developed into a pair of skeletal rods (blue) that grow to support and shape the larva. (E) The genes Alx1 and Alx4 are adjacent and are thought to have arisen from a duplication early during the evolution of echinoderms. Arrows indicate the orientation of the genes. The genes are similar, but differ in the regions of the gene that encode the protein sequence (vertical green bars). The proteins also have several identical domains, but Domain 2 (shown in turquoise), which is critical to skeleton formation, is only found in Alx1. Data for the gene organization of the purple sea urchin Strongylocentrotus purpuratus (as displayed here) has been obtained from the echinoderm genomic database ‘Echinobase’.