Embryogenesis starts out with the fertilized egg which divides or cleaves to form many smaller cells (blastomeres) without any change in total mass. The determinants distributed asymmetrically in the egg become partitioned into separate cells. During this period of cleavage, the embryo transforms from a solid sphere of cells into more of a hollow ball, with an internal fluid filled cavity surrounded by cells that cohere to form an epithelial sheet now called a blastula. But then gastrulation occurs which is the transformation of this ball into a structure with a gut. A part of the ectoderm which is the precursor to the epidermis and nervous system becomes tucked into the interior to form endoderm which is the precursor of the gut, lung and liver. Another group of cells move into the space between ectoderm and endoderm to from the mesoderm which is the precursor of muscles and connective tissues.

Gastrulation is just the first of a variety of cell movements that shape the parts of the body. Another extremely important movement is the process of where a broad central region of the ectoderm rolls up into a tube to create the which will form the brain and spinal cord.

Many proteins regulating development, especially pattern formation, are either  or signaling molecules. Cells do not differ because they contain different genes (in fact their genes are usually identical) but rather because cells express different genes. To a large extent, the instructions needed to produce a multicellular animal are contained in the non-coding regulatory DNA that is associated with each gene. The DNA may contain dozens of separate  or  that serve as binding sites for specific complexes of . This selective expression control 4 essential processes by which an embryo develops:

• proliferation: 1 fertilized cell is able to produce millions of other cells;

• differentiation or specialization:groups of cells do different things from other groups of cells. How can such differentiation come about? One way that this might occur, particularly early in development, is by asymmetric cell division in which a set of molecules is divided unequally between 2 daughter cells at the time of cell division. A good example of this is the vertebrate frog, zenopus laevis. The zenopusegg has a lower egg called the vegetal pole and an upper end called the animal pole. The animal and vegetal poles contain different mRNA molecules and other cell components which become allocated to separate cells as the egg cell divides. In the vegetal pole, for example, there is an accumulation of mRNAs coding for the gene regulatory protein VegT as well as some protein components of the Wnt signaling pathway. As a result, the cell that inherits vegetal cytoplasm will produce signals to organize the behavior of adjacent cells and form the gut. Fertilization by the sperm triggers an intracellular movement that gives the egg an additional asymmetry defining a dorsorventral (back to belly) difference. This event leads to a  based transport of the protein Dsh (dishevelled) which is a downstream component of the Wnt signaling pathway toward the future dorsal side. The subcellular region in which Dsh becomes concentrated gives rise to cells that express a dorsal specific set of genes which will generate further signals to organize the dorsoventral axis of the body. 

A more common way where cells differentiate is by exposing cells to signals from neighboring cells. For example, in inductive interaction a group of cells starts out having the same developmental potential but a signal from cells outside the group then drives one or more of the members of the group into a different developmental pathway. Usually the signal is limited in time and space so that only a subset of the cells closest to the source of the signal take on the induced character. The induction can get more complex however. In sequential induction, one group of cells might induce a neighboring group to specialize in a certain way with a subset of its cells closest to the inducer. This subset might then in turn induce its neighbors to specialize.  Induction does not even need to be a positive effect; it can be inhibitory. In lateral induction one or a group of cells specializes in a particular way by delivering a signal to neighboring cells that inhibits them from doing likewise.

A large number of developmental decisions are actually regulated by inhibitors rather than by the primary signal molecule. One such signalling molecule which uses the TGF? signalling pathway is the ligand BMPwith its associated extracellular inhibitors/modulators chordin and noggin. Chordin does not have its own receptor. Instead it is an inhibitor of BMP which is an inducer of the ventral part of the body. Inhibition of BMP by Chordin specifies the dorsal part of the body in the vertebrate frog, zenopus laevis. 

• interactions: the behavior of cells are coordinated with that of their neighbors

• movement: cells are moved and rearranged to form tissues and organs.

Much of what has been said above has come through the study of the .