Fig. 7. Circular propagation of E impulses at the margin. (A) Representation of a quartered, splayed preparation with the stimulating electrode (S) located on a radial strand in the peduncle (p) and the recording electrode (R1) on another peduncular radial strand. A second recording electrode (R2) was located on the stimulated strand close to the margin. This strand was cut between S and R2 at site a. A second cut at site b went through the ectoderm only, leaving the radial canal intact. RS, radial strand; v, velum; m, manubrium. The dotted lines represent tissues containing one or more radial and circular conduction pathways as described in the text. (B) A stimulus (asterisk) evoked both E and F events at R1 although only the E event arrived at R2. Thus the E impulse must propagate down one radial canal, around the margin and up other canals.