Any mechanical theory of the morphogenetic movements must take into account that they take place autonomously, that is, without the action of a force generated outside of the embryo. At least one of the tissues or structures involved in a movement must consume the stored chemical energy and use it so as to move, deform, and exert forces; its motion and deformation also depend on the physical constraints enforced by yolk, vitelline membrane, etc. Not all of these movements are equally complex. Ingression, for example, pertains to individual cells whereas involution is typically a coordinated deformation of a multilayer tissue. During embryogenesis, the first process involving invagination is gastrulation, that is, the transformation of a single-layer shell-like blastula into a more complex gastrula featuring three layers of cells, each developing into a specific system or part of the body. The ventral furrow is a channel-like cavity with the axis running parallel to the tissue surface.