In deuterostomes, depending on the species, there may
be a transient mesendoderm, soon separating into mesoderm
and endoderm (e.g. in hemichordates,
echinoderms and amphioxus), or endoderm and mesoderm
may be separated from the start of gastrulation (e.g.
in Xenopus). In either case, antero-posterior patterning
commences in all three germ layers around the time of
gastrulation. A given gene can be expressed in more than
one germ layer, and one germ layer can influence patterning
in others.