19The cell nuclei of Ophisthokonts, the eukaryotic supergroup defined by fungi and 20 metazoans, is remarkable in the constancy of both their double-membraned structure and 21 protein composition. Such remarkable structural conservation underscores common and 22 ancient evolutionary origins. Yet, the dynamics of disassembly and reassembly displayed 23 by Ophisthokont nuclei vary extensively. Besides closed mitosis in fungi and open mitosis 24 in some animals, little is known about the evolution of nuclear envelope break down 25 (NEBD) during cell division. Here, we uncovered a novel form of NEBD in primary 26 oocytes of the flatworm Schmidtea mediterranea. From zygotene to metaphase II, both 27 nuclear envelope (NE) and peripheral endoplasmic reticulum (ER) expand notably in size, 28 likely involving de novo membrane synthesis. 3-D electron microscopy reconstructions 29 demonstrated that the NE transforms itself into numerous double-membraned vesicles 30 similar in membrane architecture to NE doublets in mammalian oocytes after germinal 31 vesicle breakdown. The vesicles are devoid of nuclear pore complexes and DNA, yet are 32 loaded with nuclear proteins, including a planarian homologue of PIWI, a protein essential 33 for the maintenance of stem cells in this and other organisms. Our data contribute a new 34 model to the canonical view of NE dynamics and support that NEBD is an evolutionarily 35 adaptable trait in multicellular organisms. 36 37 209 2. Kutay, U. and M.W. Hetzer, Reorganization of the Nuclear Envelope during 210 Open Mitosis. Current opinion in cell biology, 2008. 20