Figure 1. Semidiagrammatic drawing of a generalized eukaryote cell. The connections between the endoplasmic reticulum, the organelles and the nuclear membrane would not necessarily exist permanently in a living cell; they have been included to show the topological arrangements of the organelles and membrane system. The cytoskeleton has been omitted for clarity.

Figure 2. (a) Transmission electron micrograph of eukaryotic cells – mouse fibroblasts. Note prominent nuclei with a few nuclear pores visible; mitochondria and endoplasmic reticulum are also present. Photo reproduced courtesy of S. Runge. (b) Trichonympha, member of the Parabasalia, a very early branch of the Eukarya. Trichonympha lacks mitochondria, but has many types of bacterial symbionts, both intracellular and extracellular. It is itself a symbiont, living in the hindgut of termites. (c) Unidentified spirochaete bacteria. These are also gut symbionts of termites. (d) Glenobotrydion from the Bitter Springs Formation of Australia, approximately 850 million years old. Note inclusions that may or may not be nuclei or organelles. (e) An acritarch, Leiosphaeridia, from the late Precambrian of the St Petersburg region, Russia. Reproduced from the University of California Museum of Paleontology (UCMP) collection. (f) Grypania spiralis from the Negaunee Dolomite of Michigan, USA, 2.1 billion years old. If this is a eukaryote, it would be the oldest currently known. Reproduced from the University of Central Arkansas (UCA) collection.

Figure 3. (a) Anabaena flos-aquae, a filamentous cyanobacterium. Note the presence of heterocysts, cells specialized for nitrogen fixation. (b) Volvox (Chlorophyta, Volvocales). A close-up of this large, spherical, coenocytic alga. Note the network of fine cytoplasmic connections between cells. (c) Coprinus hyphae (Fungi, Basidiomycetes). Note the ‘clamp connections’ between cells, typical of basidiomycete fungi. (d) Fruiting body of Dictyostelium. The cellular slime moulds produce stalks and spore masses by aggregation of free-living amoebae. (e) Hydrodictyon (Chlorophyta, Chlorococcales). This net-like structure is produced by failure of cells to separate after cytokinesis. (f) Stromatolites, the fossilized remains of cyanobacterial communities. These are approximately 600 million years old, from the lower Johnnie Formation, Nopah Range, California, USA. (g) Drawing of Proterospongia haeckelii, an extant colonial choanoflagellate with cellular differentiation. Key: a, amoeboid cell; b, dividing amoeboid cell; cy, reproductive cyst; n, nucleus. Reproduced from Bütschli O (1883–87) Klassen und Ordnungen des Thier-Reichs. Erster Band: Protozoa. Leipzig: CF Winter. (h) ‘Carbon film’ of a possible eukaryotic algal blade. Lower Cambrian of Siberia. Reproduced from the UCMP collection. (i) Charnia, one of the ‘Ediacaran’ organisms, plausibly though not unanimously interpreted as a metazoan. Late Precambrian, White Sea coast, Russia. Reproduced from the UCMP collection. (j) Aulichnites, a trace fossil produced by an unknown metazoan. Late Precambrian, White Sea coast, Russia. Reproduced from the Paleontological Institute of the Russian Academy of Sciences collection.