Materials and Methods.-Thirty developmental stages have been distinguished for this species by Arnold" 4 whose numbering system will be used. Cleavage is meroblastic. Cells with diameter of 20-50, g form a cap, the blastodisc, on the yolk cell which retains most of the mass of the original ovum (see stages 9 and 13, Fig. 1). The blastodisc envelops the yolk cell almost entirely by stage 17; the developing organs then first appear as thickenings of the blastodisc. Subsequent differentiation of the external structures is illustrated in Figure 1, stages 21, 23, 25, 27. In these later stages only the outer yolk mass can be seen in the drawings; however, a stalk of the yolk cell extends internally along the embryo making extensive contact with embryonic cells. The embryo is transparent even at late stages, permitting deep tissues to be seen.

The chorion (shown surrounding stage 9, Fig. 1) was removed with fine forceps. The exposed embryo was bathed in sea watercooled to 15-17'C. In experiments on later stages (22-27) urethan was added to a final concentration of 0.5-0.8% to reduce movement. The techniques for electrical measurements were conventional. Current was supplied to the interior of a cell by applying a potential between an intracellular microelectrode and an Ag-AgCl electrode in the bath. Resulting potential changes were recorded between another microelectrode and the same bath electrode. In many experiments a second recording microelectrode was also used.