THE RELATION BETWEEN CLEAVAGE AND TOTAL ACTIVATION IN ARTIFICIALLY ACTIVATED EGGS OF URECHIS

¹ From the William G. Kerckhoff Laboratories of the Biological Sciences, California Institute of Technology, Pasadena, California and the William G. Kerckhoff Marine Laboratory, Corona del Mar, California

1. The rate of increase in percentage activation of Urechis eggs with hypotonic sea water is shown to decrease as the concentration of sea water used is increased from distilled water to 80 per cent sea water.

2. The rate of increase in volume also decreases with increased concentration of sea water.

3. For dilutions of sea water ranging from 75 per cent to 45 per cent, the activation passes through a maximum (usually 100 per cent) and then returns to zero per cent with longer exposures. For lower concentrations of sea water the return to zero per cent is not obtained, but a high percentage of activation is maintained. With 80 per cent sea water the return to zero per cent activation also does not occur.

4. The activation-time curves for 75 per cent to 45 per cent sea water are of the form of skew distribution curves, rising rapidly to 100 per cent activation and falling more slowly to zero per cent.

5. The activation-volume curves are presented for 65 per cent to 45 per cent sea water and are of the form of a normal probability curve. Tliey are roughly identical for the various dilutions of sea water.

6. Practically every series of experiments shows an inverse relation between the percentage of total activation and percentage of cleavage (of the activated eggs); so that as the percentage of activation increases with time of exposure, the percentage of cleavage decreases, and when the percentage of activation decreases with exposure the percentage of cleavage increases.

7. The over-exposed unactivated eggs are still capable of fertilization and of producing normal embryos in spite of the fact that a shorter exposure would have resulted in their becoming activated upon return to normal sea water.

8. The variation in rate of activation with concentration of sea water, the type of activation-time curves, the activation-volume curves, and the fertilization of over-exposed eggs are shown to be interpretable on the basis of volume change occurring in the dilute sea water, a definite volume range being optimum for activation. The cleavage-activation relation is shown to be the outcome of the previously reported result that only the "poorly activated" eggs divide, and its interpretation, based also on the exposures producing such eggs, involves the assumption of a "sub-optimum volume range" on both sides of the optimum.