FURROWING IN FLATTENED SEA URCHIN EGGS

¹ Colby College, Waterville, Maine and Stazione Zoologica, Naples, Italy

1. Eggs swollen in 60% sea water cleave effectively when flattened.

2. Furrowing continues after abscission of one or both poles of the egg.

3. Furrowing is initiated and completed after hemisection of the still spherical egg through the long axis of the amphiaster.

4. Bisection of the spindle just before egg elongation causes a diastral egg to cleave into four blastomeres, two of which lack asters.

5. Islands of furrow cortex "isolated" at early furrowing shrink progressively during division. This is regarded as evidence for active furrow contraction.

6. Tubes of furrow cortex prepared from the furrow walls shrink in directions quite different from the original one.

7. The furrow cortex is not determined until the beginning moment of elongation for cleavage.

8. The position of the furrow is not necessarily related to the plane of the spindle equator.