Contents

16 December 1988
Vol 242, Issue 4885

About The Cover

Three stages in the embryogenesis of the ascidian Boltenia villosa. Upper left: fertilized egg, 30 minutes after sperm penetration. The endogenous orange pigment, which was uniformly distributed in the unfertilized egg, has formed a cap at the vegetal pole. Center: two-cell stage, 120 minutes after fertilization. Lower right: gastrula, 13 hours after fertilization. The pigment has segregated into muscle-lineage cells, which line the posterior rim of the blastopore. See page 1572. [L. Simoncini, M. L. Block, W. J. Moody, Department of Zoology, University of Washington, Seattle, WA 98195]