 Textbooks (e.g., Gilbert 2010; Slack 2013; Wolpert et al. 2010) typically describe a canonical set of events surrounding the changing structures displayed during animal  development.[1]  The first of these is fertilization (in sexually reproducing species), where an already semi-organized egg merges with a sperm cell, followed by the fusion of their nuclei to achieve the appropriate complement of genetic material. Second, the fertilized egg undergoes several rounds of cleavage, which are mitotic divisions without cell growth that subdivide the zygote into many distinct cells  (Figure 2).  After many rounds of cleavage, this spherical conglomerate of cells (now called a blastula) begins to exhibit some specification of germ layers (endoderm, mesoderm, and ectoderm) and then proceeds to invaginate at one end, a complex process referred to as gastrulation that eventually yields a through-gut. All three germ layers, from which specific types of cells are derived (e.g., neural cells from ectoderm), become established during gastrulation or shortly after it  completes.[2] Organogenesis refers to the production of tissues and organs through the interaction and rearrangement of cell groups. Events confined to distinct taxonomic groups include neurulation in chordates, whereas other events correlate with mode of development (metamorphosis from a larval to adult stage) or individual trauma (regeneration of a limb).