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

Abstract

After reporting the attachment of the sea urchin blastomeres to the surrounding hyaline envelope, the author shows that, as development proceeds, the envelope gets gradually inflated as the result of fluid accumulation within it. This accumulation is caused by an increase of the internal colloid osmotic pressure which takes place in a step-wise fashion synchronously with cleavage. A combined effect of attachment of the blastomeres to the envelope and inflation of the envelope results in achievement of the morula form. The wall of the morulae is permeable to sucrose. Concomitantly with hatching, the wall suddenly becomes impermeable to sucrose, indicating that the next means of cell association becomes effective in the biastula stage. In other words, the wall of the morula is more elastic like a rubber balloon, whose size depends on internal pressure. Owing to the closer cell association, the blastular wall is more rigid. Therefore, if the internal pressure is reduced, an indentation at one place in the wall would form. A spontaneous decrease in larval size can actually be seen immediately preceding gastrulation. When the primary mesenchyme cells migrate into the blastocoel, they detach themselves from the hyaline layer. The part of the layer thus deprived of the cell lining is pushed out as empty lobes. Many such lobes appear at the vegetal pole in this stage.