In embryos of glossiphoniid leeches such as Helobdella triserialis and
H. robusta, pairs of adjacent 'o/p' ectodermal blast cells are known
to be developmentally equipotent and yet eventually contribute distinc
t sets of 'O' and 'P' progeny to the nervous system and epidermis of t
he mature leech. It has been thought that the fate-determining interac
tions in this 'O-P equivalence group' take place between the equipoten
t cells themselves. We show here that such intra-group interactions ar
e neither necessary nor sufficient. Instead, transient contact with ce
lls in another ectodermal lineage is necessary and sufficient to induc
e o/p blast cells to assume the P fate. In the absence of this contact
they assume the O fate.