1. Sea-urchin eggs have been treated with strong colchicine during mitosis, and examined with a variety of techniques.

2. Polarized light observations show that cleavage can take place when the asters and spindle have completely disappeared, provided the cell has reached mid-anaphase, by which time the chromosome groups are well separated.

3. Observations on stained sections have confirmed those with polarized light and have also shown that the minimum separation of the chromosome groups for successful cleavage is about 5 µ.

4. Viscosity measurements have shown that colchicine destroys the gelated structure of the asters, and reduces the viscosity of the cleaving egg to about the same low value as the unfertilized egg.

5. Cycles of chromosome condensation persist in eggs treated with weak colchicine, even though cleavage has been prevented by an early application of the colchicine. These cycles are associated with changes in the rigidity and light-scattering properties of the cortex.

6. The relevance of these observations to some theories of cell division is discussed.

Now at the Department of Zoology, University of Edinburgh.