We have used whole mount in situ hybridization to analyze the pattern of expression of the gene Endo 16 in S. purpuratus embryos. The mRNA is first detectable at 18 h post-fertilization in the cytoplasm of blastomeres derived from the Veg2 6th cleavage tier. The number of Endo 16 positive cells increases gradually through the beginning of gastrulation, and these cell numbers are in agreement with estimates of the number of cells that should be in the vegetal plate at these stages. We conclude that Endo 16 expression is indeed an early vegetal plate marker and that this gene is expressed by all Veg2 tier derivatives while they are part of the vegetal plate. The progressive regionalization of Endo 16 expression that occurs in normal embryos is also seen in lithium chloride induced exogastrulae, leading to the conclusion that genetic regulation of endoderm differentiation is programmed into the vegetal plate cells once they have been specified. Finally, we report a reproducible phenomenon seen in cultures of LiCl exogastrulae, in which the tips of the everted archenterons fuse, followed by the induction of supernumerary pigment cells.