CYTO-EMBRYOLOGICAL STUDIES OF SEA URCHINS. II. BLASTULA STAGE

¹ Biology Department, Tokyo Metropolitan University, Meguro-Ku, Tokyo, and Misaki Marine Biological Station, Kanagawa-Ken, Japan

1. The accumulation of a fluid under the hyaline plasma layer of Strongylocentrotus pulcherrimus eggs, and its movement during cleavage and interkinesis are described. It is shown that an increase in volume takes place in connection with each cleavage, and that the accumulated fluid comes around to the center of the larva, giving rise to the segmentation cavity.

2. Gum arabic prevents the fluid accumulation during early cleavage periods and reduces the volume of the segmentation cavity in later stages. Raising the Ca level of the medium brings about a similar result, by causing a hardening of the hyaline plasma layer and thus preventing increase in size.

3. Moore's finding—hat at the time of hatching, the blastular wall, which has been permeable to cane sugar molecules, suddenly becomes impermeable to them—is confirmed.

4. After the migration of the primary mesenchyme cells from the blastular wall into the blastocoel, the vegetal hyaline layer which has been in contact with these cells is lifted up as folds in regular sea urchins. In the larvae of irregular sea urchins, it is dissolved away.

5. The possible significance of changes in the mode of association of the component cells on certain gross morphological changes of the sea urchin larva is discussed.