By far the greater part of biological science, quantitatively, is devoted to animals, because we are among them, and more specifically because the study of them may advance medicine. There are two great differences between animals and plants from the viewpoint of how they develop. First, animals have a precise body plan, the formation of which must be completely executed before they can properly function. Second, in their developmental processes, cells move, changing their relative positions sometimes individually and sometimes by concerted streaming of large numbers of cells together.

Both of these differences make experimental pursuit of the concepts of pattern formation more difficult in animals than in plants. The constancy of numbers of parts, such as limbs, well regulated in the face of varying embryo size, is a challenge to theory, because it shows that developmental mechanisms can do more than mark out a scale of distances, but can also adjust that scale to embryo size. For plants, I have shown by a number of examples in Part I that quantitative spatial measurements during development are very valuable in trying to correlate theory and experiment. I discuss spatial measurements in relation to Drosophila segmentation in Chapter 8; but I think that any detailed account of the phenomenon of gastrulation, which I discuss briefly in Section 9.4, will show the reader that it would be very difficult to devise a programme of quantitative spatial measurement to study the applicability of particular theories there.