Development is the process that transforms an egg into a growing embryo and eventually into an adult form.
How is this process to be understood? According to Scott Gilbert, the aesthetic of embryology separates it from other areas of biology. It is an aesthetic informed by the ordered, directional change manifest during the life of individual organisms, as they develop from a single, fertilized egg into complex patterns of diifferent, yet interacting cell, tisssues, and organs.
A few questions have dominated the study of embryology:
First of all, how is the extraordinary process of development regulated? How does a single-celled organism turn into a highly differentiated one with millions or even billions of cells? Do Genes control development? see genotype / phenotype
Is the final form of the organism set from the start? Or are there different paths of development available to the embryo? (For a discussion of preformism and epigenesis, see epigenesis}
What is the relation between the sequence of development and the process of evolution? Why do embryos of different species look so similar, and how do they end up so different?
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 (Selbst ndigkeit ) of the developing animal. (quoted in Russell, p. 115) For von Baer, development is not merely from the general to the specific, but takes place within the bounds imposed by type, (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. Von Baer criticized the concept of parallelism, later coined into the expression "ontogeny recapitulates phylogeny." While embryos of higher animal forms do ressemble those of lower, they only ressemble their embryos, not the adult forms. Thus the surest way of determining true homologies of parts will be to study their early development.
Evolutionary developmental biology (Evo Devo) has taken up the study of the evolution of embryologicial development and the study of evolution through development. For Sean B. Carroll, neither natural selection nor the molecular biology of DNA directly explain how complex visible forms are made or how they evolved.

pattern formation in fly embryo. from Carroll, Endless Forms Most Beautiful
According to Carroll, most lifeforms share a basic toolkit of genes (and the proteins they encode) that are able to be expressed in a great variety of ways through regulatory actions that take place in defined sequences and locations. The development and differences of forms depend upon the turning on and off of genes at different times and places in the course of development. Instead of having completely different genetic structures, most animals, including humans, share the same toolkit or master genes that govern the formation and patterning of their bodies and body parts. Yet despite the genetic similarity accross diverse species, vast differences in form arise from evolutionary changes in where and when genes are used, especially those genes that affect the number, shape, or size of a structure. These differences are governed by regulatory bits of DNA, about 3 percent of the total genome.
Despite the vast differences between species, Carroll notes the similarities between most body plans, which consist of modular units which are subject to wide ranges of variation within a repetitive framework. He sees this fundamentally limited typology as directly linked to the workings of the "toolkit" genes. This is why he believes that Evo Devo has given a new understanding of homology.
The Triumph of the Embryo: (from Wolpert)
After fertilization, the egg is cleaved by cell division. The first division starts with one cell, which divides into two. Shortly thereafter, another cleavage, perpendicular to the first, yields 4 cells. Cleavages continue to divide the cell into 8, 16, then 32 cells and continue until they form a hollow ball of about 1000 cells, with nutrient yolk in the center, called a blastula.
"Gastrulation is the most important moment in your life."-- Lewis Wolpert
Gastrulation is the process by which front and back, top and bottom become evident and the basic body plan is laid down. During gastrulation, the embryo forms its innermost (endoderm), middle (mesoderm), and outer layers, which will go on to form the tissues and organs found at different depths within the body. The embryo gastrulates by forming a pocket through which most of the cells originally on the outside of the embryo move inside. 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 infolding (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.) Migrating cells include neural crest cells that arise at the site where the neural tube fuses, neurons, and primordial germ cells. 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.
Neurulation is a folding of a sheet of cells on the upper surface into the neural 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.
Segmental development: repeating patterns in the early embryo include the development of the somites, paired segmenal bulges which form along the back, which subsequently form the vertebrae and the sourrounding muscles of and skin . (see serial homology )
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.
see morphic fields for background to the disputes between mechanism and vitalism.
my son Alexandros' face 4 months prior to birth