text: Another phenomena that exists in animal models is the presence of gradient fields in early development. More specifically, this has been shown in the aquatic amphibian: the newt.  These "gradient fields" as they are known in developmental biology, have the ability to form the appropriate tissues that they are designed to form when cells from other parts of the embryo are introduced or transplanted into specific fields. The first reporting of this was in 1934. Originally, the specific mechanism behind this rather bizarre phenomenon was not known, however Hox genes have been shown to be prevalent behind this process. More specifically, a concept now known as polarity has been implemented as one - but not the only one - of the mechanisms that are driving this development.
  Studies done by Oliver and colleagues in 1988 showed that different concentrations of XIHbox 1 antigen was present along the anterior-posterior mesoderm of various developing animal models. One conclusion that this varied concentration of protein expression is actually causing differentiation amongst various tissues and could be one of the culprits behind these so-called "gradient fields". While the protein products of Hox genes are strongly involved in these fields and differentiation in amphibians and reptiles, there are other causality factors involved. For example, retinoic acid and other growth factors have been shown to play a role in these gradient fields.