Different types of endoderm, including primitive, definitive and mesendoderm, 
play a role in the induction and patterning of the vertebrate head. We have 
studied the formation of the anterior neural plate in chick embryos using the 
homeobox gene GANF as a marker. GANF is first expressed after mesendoderm 
ingression from Hensen's node. We found that, after transplantation, neither the 
avian hypoblast nor the anterior definitive endoderm is capable of GANF 
induction, whereas the mesendoderm (young head process, prechordal plate) 
exhibits a strong inductive potential. GANF induction cannot be separated from 
the formation of a proper neural plate, which requires an intact lower layer and 
the presence of the prechordal mesendoderm. It is inhibited by BMP4 and promoted 
by the presence of the BMP antagonist Noggin. In order to investigate the 
inductive potential of the mammalian visceral endoderm, we used rabbit embryos 
which, in contrast to mouse embryos, allow the morphological recognition of the 
prospective anterior pole in the living, pre-primitive-streak embryo. The 
anterior visceral endoderm from such rabbit embryos induced neuralization and 
independent, ectopic GANF expression domains in the area pellucida or the area 
opaca of chick hosts. Thus, the signals for head induction reside in the 
anterior visceral endoderm of mammals whereas, in birds and amphibia, they 
reside in the prechordal mesendoderm, indicating a heterochronic shift of the 
head inductive capacity during the evolution of mammalia.