What parts of the development of vertebrates are likely to have important relationships to the development of plants or of lower animals, and what parts seem to be so intrinsically different that no parallels need be drawn? I often hear talks in which the speaker refers in apparent generality to ‘cells’ or ‘tissues’ and I keep wanting to add ‘animal’ as a preceding quasi-adjective, because what is being presented does not apply to my preoccupation with plants. First, as already mentioned, the big contrast for many events is that in much of animal development, vertebrate and invertebrate, cells move past each other to change their relative individual positions, and also often move in orderly streams to position an aggregate within the organism. The latter clearly needs consideration of mechanical forces. But when cells move individually, these movements can be directionally random (or ‘passive’ in biological terminology) and must then obey a diffusion equation. In that case, the dynamics can become a larger spatial scale analogue of reaction–diffusion, one of the things I earlier referred to as ‘cell-as-molecule’ mechanisms (Harrison 1993). In Section 7.2.1 I described Malcolm Steinberg's work on cell sorting, illustrating such passive movement, but also a patterning that involves approach to thermodynamic equilibrium rather than departure from it.