Figures 5–8: Transmission electron micrographs of cells of Ulva mUtabilis 48 h after the induction of gametogenesis. (5) The cell is polarised with most of the cytoplasm and organelles gathered at the tip area. The external cell wall has formed a projection towards the apical region. The cell wall appears differentiated at this area (N: nucleus). Scale bar: 2 µm. (6) Detail of the apical wall of the previous image, showing the loose structure of the wall and the formation of amorphous dark-stained material. Scale bar: 0.35 µm. (7) More advanced stage of a wall projection. The dark-stained material covers the projection, while its central part appears as a plug, consisting of amorphous material. The internal wall layer under the tip is considerably thickened and pushes the plug out (asterisk). Scale bar: 4 µm. (8) Cross-section of a cell with a plug-like cap. The dark-stained material is extended covering the cap. Scale bar: 0.33 µm.