This represents the ancestor to all the animals. Animals have several features that are not found in other kingdoms and that provide evidence that animals are each others' closest relatives. All are multicellular, and their multicellular bodies develop through a process of embryonic development in which at least two (in many groups, subsequently three) initial layers of tissue arise. Their bodies then develop further as these tissue layers become specialized for particular funtions.
The basic stages of embryogeny, from a single cell, the fertilized egg, through a two-layered embryo called a gastrula, which has the beginnings of a digestive tract, are shown in the following figure.
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