ABSTRACT

The other aspect of individuation is the formation of three-dimensional structures. For instance, the hollow sphere of the blastula undergoes the process of gastrulation and thus acquires a new and defmite configuration; or again, a neural plate rolls up into a neural tube, which is characterised by the well-defined swellings of the brain vesicles, etc. All such processes are 'morphogenesis' in the strict sense, since that word really means the development of shape. The shapes of organs and of the body as a whole continue to change throughout most oflife owing to the unequal growth of different parts. Such processes of relative growth have been considered in Chapter XIII; they may be considered as secondary morphogenesis. What we shall be concerned with in this chapter are the processes of primary morphogenesis, by which the original shape of the organ rudiments is first brought into being. (The distinction between these two categories of primary and secondary morphogenesis is not

sharp, but it is a useful rough classification and we shall see that in primary morphogenesis there are many factors which play a more important part than differences in growth rate.)

The two aspects of individuation-morphogenesis and pattern formation-are obviously closely connected with one another. It is hardly to be supposed that any complicated three-dimensional structure will develop unless the material out of which it is made has already developed a pattern of different properties in its various parts. Thus some degree of pattern formation probably always precedes any but the very simplest morphogenetic processes. Contrariwise it is to be expected that a developing pattern will be influenced by the shape of the area or mass in which it is forming and we shall find examples which demonstrate that this is the case. It is, however, helpful to use the distinction between pattern formation and morphogenesis as a means of arranging the subjects which require discussion into some sort of order. Moreover there is a certain difference in the kind of processes which must be involved in the two classes of phenomena. Pattern formation can, and frequently does, go on within a mass whose overall shape does not change. It requires the postulation of forces of an essentially chemical or physico-chemical order -diffusion, facilitated synthesis and the like. Morphogenesis, on the other hand, involves the actual movement of masses from one spatial position to another, and requires the intervention of physical forces such as those of surface tension, attraction, contraction, expansion, etc.