METAMORPHOSIS OF CALCAREOUS SPONGES .2. CELL REARRANGEMENT AND DIFFERENTIATION IN METAMORPHOSIS

The free-swimming larva of the caleareous sponge turns into a sessile juvenile during metamorphosis. Electron microscopic observations of me tamorphosing larvae reveal the rearrangement and differentiation of la rval cells. About 12 h after the larvae were released from a mother sp onge, the settled larvae without flagella consist of an inner cell mas s and an enveloping layer of pinacocytes. The inner mass cells have re sidual flagellar rootlets which clearly show the origin of the cells. On the other hand, the pinacocytes still show the intracellular profil e characteristic of the granular cells of the swimming larva. One day after release, scleroblasts and other mesohyl cells differentiate in t he peripheral region of the inner cell mass. Two days after release, t he central cells of the inner cell mass differentiate into choanocytes . Three days after release, a large gastral cavity is formed and lined by a layer of choanocytes. These results demonstrate the cell lineage in the metamorphosis of the calcareous sponge, that is, the pinacocyt es of a young sponge are derived from the granular cells of the swimmi ng larva and the choanocytes from the flagellated cells.