Abstract
An ultrastructural study of late-stage androgonial cells of Blasia pusilla, a thallose liverwort, showed the nearly spherical nuclei often lying close or appressed to the cell walls. In some cells the two membranes comprising the nuclear envelope separated, the inner membrane continuing intact as a limiting boundary of the nucleus and the membrane on the outer, cytoplasmic side recurving away from the nucleus to continue without evident interruption around the periphery of the cell as the plasma membrane. It is believed that Blasia offers the first completely convincing demonstration of the heretofore problematic continuity of cytoplasmic membranes. A possible sequence of events leading to this unusual relationship between nucleus and cytoplasm is suggested. The sequence includes blebbing of the outer membrane of the nuclear envelope and subsequent membrane proliferation, apparent isolation of cytoplasmic ground substance, fusion of internal membrane with the ectoplast, and migration that finally brings the nucleus into flat contact with the wall. While this manifestation of membrane continuity may be anomalous, it is not presently considered the result of cell injury.