Differences in patterns of embryonic development have until recently provided many of the important clues to animal phylogeny. Analyses of gene sequences, however, are now showing that some developmental patterns are more evolutionarily variable than previously thought. Here we describe the basic developmental patterns that vary among the major animal clades.

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The early cell divisions of an embryo are known as *cleavage. Several different patterns of cleavage exist among animals. Although these patterns can be useful for characterizing major animal groups, genomic analyses have shown that many changes have occurred in cleavage patterns throughout animal evolution.

Cleavage patterns are influenced by the configuration of the yolk, the acellular nutritive material that nourishes the growing embryo. The eggs of many animal groups contain a small amount of yolk that is evenly distributed throughout the egg cytoplasm. In some of these groups, the zygote and its descendant cells divide completely and evenly in a pattern known as radial cleavage. Radial cleavage is thought to be the ancestral condition for the bilaterian animals, as it is widely distributed among the major lineages. Spiral cleavage—a complicated permutation of radial cleavage—is found among many lophotrochozoans (a group that includes earthworms and clams). Lophotrochozoans with spiral cleavage are thus sometimes known as spiralians. The early branches of the ecdysozoans (molting animals, such as insects and nematodes) have radial cleavage, but most ecdysozoans have an idiosyncratic cleavage pattern that is neither radial nor spiral in organization (see Figure 43.3C). In reptiles, the presence of a large body of yolk within the fertilized egg creates an incomplete cleavage pattern in which the dividing cells form an embryo on top of the yolk mass.

Distinct layers of cells form during the early development of most animals. These cell layers differentiate into specific organs and organ systems as development continues. The embryos of diploblastic animals have two cell layers: an outer ectoderm and an inner endoderm. Embryos of triploblastic animals have, in addition to ectoderm and endoderm, a third distinct cell layer, mesoderm, which lies between the ectoderm and the endoderm (see Figure 43.6). The existence of three cell layers in embryos is a synapomorphy of the triploblastic animals (which form a clade), whereas the diploblastic animals (ctenophores, placozoans, and cnidarians, which are not a clade) exhibit the ancestral condition (see Figure 30.1). Some biologists also consider sponges to be diploblastic, but since they do not have clearly differentiated tissue types or embryonic cell layers, they are usually considered to be monoblastic.

During early development in many animals, in a process known as gastrulation, a hollow ball one cell thick indents to form a cup-shaped structure. The opening of the cavity formed by this indentation is called the blastopore (Figure 30.4). The process of gastrulation is detailed in Key Concept 43.3; the point to remember here is that the overall pattern of gastrulation immediately after formation of the blastopore divides the triploblastic animals into two major groups:

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Although the developmental patterns of animals are more varied than suggested by this simple dichotomy, sequencing data indicate that the protostomes and deuterostomes represent distinct animal clades. Together these two groups are known as the bilaterians (named for their usual bilateral symmetry), and they account for the vast majority of animal species.