 Several key processes underlie these distinct developmental events and the resulting features of form that emerge (e.g., the through-gut formed subsequent to gastrulation or the heart formed during organogenesis). These are critical to the ontogeny of form and link directly to major research questions in developmental biology  (Section 2).  First, cellular properties, such as shape, change during ontogeny. This is a function of differentiation whereby cells adopt specific fates that include shape transformations  (Figure 3).  Second, regions of cells in the embryo are designated through arrangement and composition alterations that correspond to different axes in different parts of the embryo (e.g., dorsal-ventral, anterior-posterior, left-right, and proximal-distal). The successive establishment of these regions is referred to as pattern formation. Third, cells translocate and aggregate into layers (e.g., endoderm and ectoderm, followed by the mesoderm in many lineages) and later tissues (aggregations of differentiated cell types). Fourth, cells and tissues migrate and interact to produce new arrangements and shapes composed of multiple tissue layers with novel functions (i.e., organs). These last two sets of processes are usually termed morphogenesis (Davies 2005) and occur via many distinct mechanisms  (Section 1.3).  Fifth, there is growth in the size of different form features in the individual, remarkably obvious when comparing zygote to adult, although proportional change between different features (allometry) is also striking.