A cyto‐embryological study of gastrulation in the sand dollar, Scaphechinus mirabilis

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Abstract
Processes of gastrulation in the sand dollar Scaphechinus mirabilis were compared with those in the sea urchin Hemicentrotus pulcherrimus, which seemed to show a typical pattern of gastrulation. Measurement of the archenteron length clearly demonstrated that invagination processes in H. pulcherrimus are divided into two phases, the primary and secondary invagination. On the other hand, invagination in S. mirabilis was revealed to continue at a constant rate. To see the movement of cells during gastrulation, embryos were labeled with Nile blue. In H. pulcherrimus embryos, labeled cells were observed along the full length of the archenteron, if the embryos had been labeled before and during the primary invagination. Labeled cells were never observed in the embryos stained after the primary invagination. In contrast, labeled cells were always discerned at the basal part of the archenteron in S. mirabilis, even if the embryos were stained after invagination had undergone considerable progress. The number of cells in the archenteron of S. mirabilis embryos increased with the advancement of gastrulation, while the numbers were almost constant in H. pulcherrimus. These results suggest that the cellular basis of gastrulation in S. mirabilis is quite different from that in well‐known species of sea urchins.