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.

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Spherical blastomeres of starfish embryos begin to adhere to neighboring blastomeres and to become columnar in shape from the 7(th) or 8(th) cleavage onward. Studying development of embryos in the presence of LiCl, we found that developmental changes in cell-cell contacts were accelerated by LiCl. In order to learn why LiCl increased the adhesiveness between blastomeres, the negative surface charge density was estimated by the method of cell electrophoresis. It turned out that the electrophoretic mobility (EPM) of all blastomeres isolated from LiCl-treated embryos before the 512-cell stage was remarkably decreased. At the mid-gastrula stage, however, when constituent cells were connected with each other more tightly, the EPM was significantly retarded irrespectively whether the cells had been isolated from control or from LiCl-treated embryos. From these results of cell electrophoresis we conclude that reduction of the negative surface charge density may be one of the important factors that enhance the adhesion of starfish embryonic cells.

Blastomeres of starfish embryos begin to increase in adhesiveness after the eighth cleavage and form a monolayered hollow blastula. To investigate factors that affect the timing of the adhesiveness increase, we changed the volume of the cytoplasm or the ploidy of embryos and examined the morphologic changes in the descendent blastomeres during early cleavage stages. In parthenogenetic embryos, in which the ploidy is doubled, the timing of the increase in adhesiveness was accelerated by one cell cycle. In contrast, the timing was delayed by approximately one cell cycle in a large-sized embryo formed by the fusion of an egg and a non-nucleate egg fragment. These two sets of observations are in accord with the expectation from the classical concept that the DNA: cytoplasmic ratio may direct the timing of events in early development. However, observations of small-sized embryos with a reduced amount of cytoplasm were contradictory to the expectation based on the DNA: cytoplasmic ratio; the timing of the increase in adhesiveness in half-sized embryos was almost the same as in control embryos and the timing was delayed by only one cell cycle in quarter-sized embryos. Measurement of the diameters of nuclei showed that the size of nuclei was variable, depending on the stage of development, the volume of cytoplasm and ploidy. We calculated a volume ratio of nucleus to cytoplasm (N: C volume ratio) for tetraploid, large-, half- and quarter-sized embryos. We found that the embryonic cells begin to adhere always when their N: C volume ratio reaches 0.06. A plausible model for the cellular timing mechanism of cell contact is proposed.

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