Generally speaking, the endoderm gives rise to the lining of the digestive tract (including the stomach, intestines, liver, and pancreas), as well as to the lining of the trachea, bronchi, and lungs of the respiratory tract, along with a few other structures.
Animals with three tissue layers are called triploblasts.Įach of the three germ layers is programmed to give rise to specific body tissues and organs, although there are variations on these themes. More complex animals (usually those with bilateral symmetry) develop three tissue layers: an inner layer (endoderm), an outer layer (ectoderm), and a middle layer (mesoderm). The four clades considered to be diploblastic have different levels of complexity and different developmental pathways, although there is little information about development in Placozoa. These animals are called diploblasts, and have a nonliving middle layer between the endoderm and ectoderm (although individual cells may be distributed through this middle layer, there is no coherent third layer of tissue). The animals that display radial, biradial, or rotational symmetry develop two germ layers, an inner layer ( endoderm or mesendoderm) and an outer layer ( ectoderm). Animals develop either two or three embryonic germ layers (Figure 1). Recall that these germ layers are formed during gastrulation, and that each germ layer typically gives rise to specific types of embryonic tissues and organs. Most animal species undergo a separation of tissues into germ layers during embryonic development. Compare and contrast the embryonic development of protostomes and deuterostomes.
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