Fig. (12).

Early developmental role of HP transfer: a hypothesis. A. Vertical induction of positional information between two layers, for example the mesoderm and the neurectoderm. In this scheme HPs expressed in one layer are transported into an abutting layer and induce compartmentalization. B. Planar induction whereby the protein induces its own expression in abutting cells and thus spreads and increases the size of its own territory. C. A boundary can form where two "infectious" HPs with self-activating and reciprocal inhibitory activities meet.