Embryonic cells make choices that guide the differentiation of their clonal descendants. Such decisions establish the outline of a snail's whole body in a spatially patterned array of just twelve diverse, unambiguously identified single cells which are differentially exposed to a signal briefly emanating from a point source. We want to know if (and how) a change in this pattern causes the neck-founder cells to inappropriately form trunk structures, and if lack of a neck makes the head pop off.