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		The effect of lithium chloride on Lytechinus variegates embryos T.J. Unger, Franklin & Marshall College and L. A. Fety, Swarthmore College Purpose This experiment will study effects of lithium chloride on sea urchin development, forcusing on archenteron formation. Introduction Gastrulation is extensive cell rearrangement where cells undergo dramatic movements and change relative positions. From this ordered movement, layers of cell are created. The cells that will form the endodermal and mesodermal layers, and then organs, are brought inside the embryo, while the cells that will form the skin and nervous system spread over the outside surface. The three germ layers - outer ectoderm, inner ectoderm, and interstitial mesoderm - are produced during gastrulation (Gilbert, 1997). After the sixth cleavage in normal sea urchin, two tiers of eight cells each are formed in the vegetal half of the embryo. The top tier is termed veg1 and the bottom tier is termed veg2. Veg1 lineages have been shown to contribute portions of the definite hindgut, midgut, and the ectoderm that surrounds the blastopore at the completion of gastrulation (Cameron, 1997). Veg2 and micromere lineages normally contribute to secondary mesenchyme, the coelomic sacs, as well as most of the archenteron (Cameron, 1997). High levels of B-catenin in micromeres prior to gastrulation suggest that B-catenin plays a signaling role (Miller and McClay, 1997).