Abstract

For a multicellular organism to proceed from egg to adult it must: (i) undergo cell division, (ii) differentiate, (iii) remain a unified whole (Ho). These requirements are at right angles to each other. The first two are achieved through hierarchical processes (vertical control) that are relatively well understood, the third through non-hierarchical processes (horizontal control) physiological evidence for which is abundant, though not widely recognized as a form of control. The essay gives an example of a tissue – the skin of a living squid – whose horizontal network properties come to light when nervous (vertical) control is removed. It offers the name homeotaxy or ‘peer conformity’ for the general principle (allied to the community effect, Gurdon 1988) that constrains the parts of the whole to be in the same state within any given layer of the network – where layers correspond to ontogenetic stages in the development of the tissue – and discusses the question of a need and a name for this principle in Biology.