ACCELERATION OF CLEAVAGE OF ARBACIA EGGS BY HYPOTONIC SEA WATER

¹ Marine Biological Laboratory, Woods Hole, and Department of Zoology, University of Michigan, Ann Arbor

1. In sea water diluted to a concentration 98 per cent to 94 per cent that of normal sea water, the eggs that responded showed acceleration of the first cleavage as high as 5.1 per cent of the normal time between insemination and cleavage. Eggs from some sea urchins did not respond, but none showed deceleration in this range of concentrations.

2. In concentrations 92 per cent to 88 per cent, some sets of eggs were accelerated, while others were retarded. This reveals a threshold of antagonism between accelerating and decelerating effects.

3. Concentrations 84 per cent and less always retarded, as did concentrations hypertonic to sea water.

4. Sea water diluted with isotonic electrolyte or nonelectrolyte did not produce an acceleration.

5. The response could be obtained with treatment begun shortly after entrance of the sperm, or as late as the diaster of the first cleavage. The latter indicates that the acceleration results in part, at least, from action upon some phase of mitosis.

6. The second cleavage could be accelerated separately, but eggs left in hypotonic sea water did not show cumulative accelerations for each cleavage. This shows that limited energy is available, or the effect is of the nature of a stimulus.