Video S1 Animated view of rendered bronchi and alveoli. This is the same 3D model shown in Figure 3C . The rotation of the model permits the viewers to observe the lung from different angles. The model can be cut from any angle in order to give a detailed view of the inner part of the lung. The bronchi, bronchioles, and alveoli are all visible. (AVI) Video S2 The virtual endoscope from the bronchi to one alveolar duct. This is the same model shown in Figure 3D . The volume data of the lung airways is separated from CT slices using 3D image segmentation method. 360u rotation of the model displays the spatial relationships of different lung airways. And the virtual endoscope of the model reveals the internal lung airways. (AVI) Evolution of size and pattern in the social amoebas A fundamental goal of biology is to understand how novel phenotypes evolved through changes in existing genes. The Dictyostelia or social amoebas represent a simple form of multicellularity, where starving cells aggregate to build fruiting structures. This review summarizes efforts to provide a framework for investigating the genetic changes that generated novel morphologies in the Dictyostelia. The foundation is a recently constructed molecular phylogeny of the Dictyostelia, which was used to examine trends in the evolution of novel forms and in the divergence of genes that shape these forms. There is a major trend towards the formation of large unbranched fruiting bodies, which is correlated with the use of cyclic AMP (cAMP) as a secreted signal to coordinate cell aggregation. The role of cAMP in aggregation arose through co‐option of a pathway that originally acted to coordinate fruiting body formation. The genotypic changes that caused this innovation and the role of dynamic cAMP signaling in defining fruiting body size and pattern throughout social amoeba evolution are discussed. BioEssays 29:635–644, 2007. © 2007 Wiley Periodicals, Inc. The social amoebas or Dictyostelia represent one of nature's several independent inventions of multicellularity. The dictyostelia are members of the amoebazoans, a genetically highly diverse group that is the closest sister group to the clade containing the animals and fungi. (1) Except for the myxomycetes, all other known amoebazoans are microscopic unicellular organisms. The myxomycetes alternate a trophic amoeboid stage with a syncytial form. Here, a single cell with millions of nuclei, can grow up to several meters across. (2) Social amoebas also have a trophic amoeboid stage, but they achieve macroscopic dimensions by aggregation. (3) This occurs in response to starvation, which triggers regulated secretion of chemoattractant by the amoebas (Fig. 1) . Cellular agglomerates are formed, which can consist of up to a million amoebas. Sophisticated cell-cell signalling mechanisms between the amoebas orchestrate the differentiation of up to five different cell types and coordinate an intricate progression of cell movements. In combination with the synthesis of a flexible skin-like matrix, cell differentiation and cellular movement first generate the formation of a motile structure, called the ''slug''. The slug responds to chemical gradients and to light and warmth, which cause it to move to the soil's top layer. Here, the slug projects upwards and forms the fruiting body. This again involves highly ordered movement and differentiation and yields a slender column of stalk cells that bears aloft a global mass of spores. Depending on the species, the stalk can show different patterns of side branches and/or be decorated with disc, root or cup-shaped support structures (Fig. 2) . Unlike the ontogeny of sessile organisms like plants and fungi, which depends largely on series of directional cell divisions, the formation of fruiting bodies in social amoebas is more similar to the ontogeny of animal form. Both depend strongly on an intertwined program of cell movement and cell differentiation.