Memoirs: Studies in Dedifferentiation. IV. Resorption and Differential Inhibition in Obelia and Campanularia

Abstract

1. Confirmation is given of the results of Loeb, Thacher, Godlewski and Gast, and others, in showing that the hydranths of hydroids (in this case Obelia and Campanularia) when exposed to unfavourable conditions proceed to dedifferentiate and to be resorbed, wholly or mainly, into the stem. 2. Exposure to toxic agencies accelerates the process. Too great concentration of poison kills the zooids before dedifferentiation starts. Below the death-point, the acceleration is proportional to the concentration. 3. The effect is non-specific, both KCN and HgCl2 producing the same result as prolonged exposure to laboratory conditions. 4. When zooids are separated from the stem, resorption is impossible. Dedifferentiation, however, proceeds until an ovoid undifferentiated body packed with cells is produced. 5. The tentacles are first affected, then the hypostome. In early stages, separate tentacles may fuse locally. Stumps of tentacles are, however, still present after the hypostome has quite disappeared. The body becomes ovoid, then spherical, and is finally reduced to a minute pigmented dot. 6. The surface tension of the dedifferentiated zooid causes the emigrated zooid cells to flow into the stem. In later stages spontaneous pulsations of the stem and of the zooid (these possibly not spontaneous) occur. 7. Dedifferentiation of the tissues of the tentacles starts at the tip. Progressive histological dedifferentiation of the endoderm cells can thus be clearly followed in a single section. 8. Only after the mesoglaea at the base of the tentacle has ruptured can the contents be resorbed (confirming Thacher). 9. Cnidoblasts with nematocysts can be distinguished within the gastrovascular cavity as resorption proceeds. They may also be seen phagocytosed within large cells, presumably immigrated endoderm cells. 10. The dedifferentiation is regarded as due to interference with general metabolic processes, and especially with the production of the energy needed to maintain form against surfacetension. 11. Resorption is regarded as the natural result of dedifferentiation when there are adjacent cavities into which the cells can migrate. In higher forms it has been largely replaced by phagocytosis.