[Electron microscoic study on the regeneration of the cerata].

The regeneration of the cerata was studied in Aeolidiella soemmeringi (Leuckart, 1828) by light and electron microscopy. After amputation of the tips of the cerata the edges of the epidermis and of the hepatopancreas are drawn together by muscular contraction. The restitution of the cnidosac and of the hepatopancreas begins within groups of reserve cells, which are to be found in the hepatopancreas at each time. The restitution of the epidermis starts from undifferentiated replacement cells in the epidermis. Undifferentiated cells, which probably are fibroblasts and amoebocytes form a loose, spongy blastema. The amoebocytes become more and more loaded with phagocyte material and often show a pycnotic nucleus with very densely packed chromatin, while the fibroblasts predominantly form the fibrocytes of the mesenchym of the growing ceras. Thus it seems, that to a certain extent each tissue, i.e. ectoderm, entoderm and mesoderm, retains its individuality and is repaired from its own elements.