Initiation and early patterning of the endoderm.

We review the early stages of endoderm formation in the major animal models. In Amphibia maternal molecules are important in initiating endoderm formation. This is followed by successive signaling events that establish and then pattern the endoderm. In other organisms there are differences in endodermal development, particularly in the initial, prephylotypic stages. Later many of the same key families of transcription factors and signaling cassettes are used in all animals, but more work will be needed to establish exact evolutionary homologies.