In previous chapters we found that, by virtue of their internal dynamics, cells could assume distinct states and thereby follow alternative developmental pathways. This process ultimately generated various types of terminally differentiated cells (Chapter 3). We also found that tissues made up of multiple cells, linked together directly (Chapters 4 and 5) or via an extracellular matrix (Chapter 6), could undergo alterations in shape and form (morphogenesis), leading to the development of new structures. Although developing tissues need not contain different cell types in order to undergo morphogenesis, differentiation often sets this process in motion. For example, sorting and tissue engulfment, which together comprise one of the classes of morphogenetic phenomena discussed earlier, require at least two populations of cells that are differentiated with respect to their cell-surface adhesivity.
Cell sorting, however, is unusual among the morphogenetic processes dependent on differentiation in that the initial differentiation event need not be spatially controlled. Recall that the random mixing of two cell types and the fusing of fragments of the tissues from which they were derived ultimately achieved the same morphological outcome (Fig. 4.5). Thus, if differentiation in a developing cell mass occurs in a spatially random manner, so that two cell types with differing adhesive properties come to be dispersed in a salt-and-pepper fashion, the final configuration of the eventually phase-separated tissues will always be the same. A case in point is Hydra, a diploblastic, i.e., two-germ-layered, organism (see Chapter 5), which starts out as a single-layered blastula.