In the previous chapters we have followed the process of rapid cell divisions in the early embryo until the formation of the blastula, initially a solid mass of cells poised to develop into the structurally and functionally differentiated organism. Adhesive differentials along the surfaces of individual cells of the early blastula, the blastomeres, drive the formation of spaces or lumens (see Fig. 4.2) within the embryos of most species. As a result, the typical blastula acquires a geometrically simple closed spheroidal structure that consists of a single cell layer enclosing the hollow blastocoel.

By the time the blastula has developed, the embryo already contains, or begins to generate, a number of differentiated cell types (see Chapter 3). Insofar as these cell types have or acquire distinct physical (adhesive, contractile) properties, compartmentalization or other forms of regional segregation start taking place. This regionalization, accompanied by the collective movement of the resulting cell masses, gives rise in most cases to embryos consisting of two major cell layers, referred to as “germ layers,” along with some subsidiary populations of cells.

The various modes of cell rearrangement by which a solid or single-layered blastula becomes multilayered are known collectively as gastrulation. In “diploblastic” animals, such as sponges and coelenterates (hydra, jellyfish), gastrulation is complete when the two germ layers, the outer ectoderm, and inner endoderm, are established. Further cell specialization occurs within these two main layers and any subsidiary cell populations.