"Gastrulation is the most important moment in your life."-- Lewis Wolpert
For the division between issues of heredity and issues of development, see genotype / phenotype
for discussion of preformism and epigenesis, see epigenesis
see morphic fields for background to the disputes between mechanism and vitalism.
Karl Ernst von Baer was recognized as the founder of embryology even by his contemporaries. For von Baer, development is essentially a process of differentiation, (Ausbildung ) from the homogeneous to the heterogeneous, by which the germ becomes ever more and more individualized. "The essential result of development," he wrote," when we consider it as a whole, is the increasing independence (Selbständigkeit ) of the developing animal. (quoted in Russell, p. 115) Von Baer criticized the concept of parallelism, later coined in the expression "ontogeny recapitulates phylogeny." For von Baer, development is not merely from the general to the specific, but takes place within the bounds imposed by type, (which he defined as "the positional relationship of the inherent elements and the organs." in which characters of the larger classificatory group appear before those of the species and individual. And if embryos of higher animal forms ressemble those of lower, they only ressemble the embryos, not the adult forms. Thus the surest way of determining true homologies of parts will be to study their early development.
The Triumph of the Embryo:
After fertilization, the egg is cleaved by cell division into a hollow ball of about 1000 cells called a blastula.
Gastrulation is the process by which front and back, top and bottom become evident and the basic body plan is laid down. First, some cells leave the wall and enter the hollow interior. These cells will later become the skeleton. Then the spherical blastula is transformed into a torus, as an in folding (or invagination) of the wall moves completely accross the interior to form the gut. The point of infolding becomes the anus, and the other end becomes the mouth. One of the mechanisms of this movement is the extension and contraction of filopodia, fine extensions from the cell with muscle-like filaments within. The filopodia adhere to the wall, which provides a template for development, as its surface shows (changing) patterns of adhesiveness and the filopodia find the most stable contact regions.
Cell adhesion, based on cell adhesion molecules (CAMs) on the cell surface, is one of the primary mechanisms of self-assembly. Contractions, changes in adhesion, cell movement, and growth are the principal cellular activities that go the mould the form of the embryo. (According to Gerald Edelman, the primary cellular processes are, division, migration, death, adhesion, and induction.)
According to Wolpert, a generative programme -- the changing pattern of cell contractions and cell contacts -- provides the instructions for making the shapes. He compares it to Origami, consisting of a set of instructions for folding and unfolding, which can lead to a very complex final form.
Neurulation is a folding of a sheet of cells on the upper surface into a tube that will develop into the brain and spinal cord. The lens of the eye is formed in a similar manner. Neurulation takes place through embryonic induction and is the result of signals passing from a set of cells in one layer to a set of cells in another. This place-dependent or "topobiological" differentiation sets position-dependent cues for future induction events.
Migration of cells is another important phenomenon. Migrating cells include neural crest cells that arise at the site where the neural tube fuses, neurons, and primordial germ cells.
Segmental development: repeating patterns in the early embryo include the development of the somites, which subsequently form the vertebrae and the muscles of the back.
Growth: is an important mechanism for generating changes in form. The face, for instance, starts off as a series of bumps, called processes, each with its own characteristic growth pattern that together generate the face.
Induction: In 1924, Hans Spemann proved that a graft could induce host tissues adjacent to it to completely change their fate and to form a second embryo in relation to the graft, which he called the organizer.