Timing of gastrulation in fused double-embryos formed from eggs with different cleavage schedules in the starfish, Asterina pectinifera.

The aim of this study is to explore the control mechanism which regulates the timing of gastrulation. We wished to know whether the timing is rigidly determined within the constituting cells of the embryo of whether it is changed by intercellular communication systems. Starfish-fused double-embryos were formed by contact between pairs of denuded eggs with different cleavage schedules, one egg in each pair being distinguished with vital stain. Electron microscopy revealed identical ultrastructural characteristics in every cell contact: Septate junctions were found not only between cells in the contact area between the two counterparts constituting a fused embryo but also between cells within a counterpart. In fused embryos formed from stained eggs and unstained eggs with the former having started the first cleavage 30 min earlier than the latter, the stained counterparts began to invaginate about 30 min earlier than the unstained. When fused embryos were produced by putting stained eggs in contact with unstained eggs which had begun the first cleavage 2 hr later than the stained eggs, gastrulation of the stained counterparts took place about 2 hr earlier than that of the unstained counterparts. These results suggest that the timing of gastrulation is fixed within the cells of an embryo and cannot be altered in spite of the cell-to-cell communication that appears to exist.