Ultrastructure of meiosis and centriole behaviour in Ulva mutabilis Foyn.

The present work deals with the ultrastructure of the meiotic processes leading to the formation of zoospores. The formation of an exit pore on the outer surface of the sporangium is the first visible sign that the cell will undergo meiosis and not just a somatic division. Prophase I nuclei differ from nuclei in mitotic prophase by having one or several invaginations of the nuclear membrane. Electron-dense lines probably representing the lateral elements of the 8ynaptinemal complex are observed during prophase I. The central element of the complex has, however, never been observed. The fate of the centrioles during meiosis is described in detail. The number of duplications of the centrioles is found to be the same at meiotic and mitotic zooid formation. The number of cell divisions is, however, different in the 2 cases.