RADIALLY ORIENTED CLEAVAGE IN BLASTULAE AND IN GASTRULAE OF THE STARFISH, ASTERIAS FORBESI

¹ Biology Department, Brooklyn College, Brooklyn, N. Y.

Blastulae and gastrulae of the starfish, Asterias forbesi, were fixed, run into paraffin, sectioned, and stained to show the mitotic figures. These were found to lie in a tangential direction, thus leading to radial cleavages in the blastulae, and to their equivalent (i.e., cleavage perpendicular to a tangent to the curved surface) in the gastrulae. The single layer of cells characterizing these stages is therefore the result of spindle figure and of cleavage plane orientations. Other observations suggested that the spindles may lie in definite arrangements relative to each other. Several patterns were found, and their relationship to blastula and to gastrula formation is briefly discussed. There were indications that the blastomeres may, though rarely, undergo cleavage along a longer diameter.