Specific Cell Ligands and the Differential Adhesion Hypothesis: How do they fit together?

At present there are two different formulations — the specific ligand hypothesis and the differential adhesion hypothesis — each of which purports to ‘explain’ spontaneous cell sorting and related phenomena. These two explanations appear to be cast at right angles to one another at best, or at worst to be altogether non-intersecting. In actuality, however, they are directed toward two different aspects of the question: ‘How are the phenomena of multicellular assembly determined?’ It is my purpose here to lay these relationships out as completely as I can. To do so, I must first review in some detail the differential adhesion hypothesis (DAH) and the evidence supporting it. Then it will be quite easy to see how the DAH and specific (and also not-so-specific) ligands fit into the same picture.