The cellular reaction is characterized by migration of the cell boundary. There are, however, two states in which a cell boundary is stationary; the first corresponds to the beginning of the precipitation reaction, and the second to the ending of the dissolution reaction. Specimens of lead-rich Pb-Sn alloy have been used to investigate these two terminal states of the reaction. We illustrate in the case of the beginning state what initiates the motion of the cell boundary, and in the case of the ending state how static rather than kinetic analysis can be applied. © 1972 The Metallurgical of Society of AIME.