All embryos of a given species have a similar structure or body plan. 
How does it occur?
During development each cell must differentiate according to its position in the embryo, so that the "correct" cell types arise in the correct place. In other words, cells must know where they are in relation to other cells in the embryo.This is achieved in the developing embryo by giving each cells a positional value in relation to the main embryonic axes. Regional specification describes any mechanism that tells a cell where it is in relation to other cells in the embryo, so that it can behave in a manner appropriate for its position. 

How cells become aware of their position? What is the nature of the positional information?
Several model systems indicate that cells may acquire positional values on the basis of their distance from a source of a morphogen. A morphogen is a secreted substance that can influence cell fate having different effects at different concentrations (.i.e. to be able to specify multiple cell fates at once!). The positional information along an axis can be generated for example by the synthesis of a morphogen at one end of the axis, and by its diffusion away from the source, setting up a gradient. Cells at different position along the axis would receive different concentrations of the morphogen and this would induce different patterns of gene expression at different concentration thresholds. Such concentration-dependent patterns of gene expression would represent the positional identity of the cell. Besides the concentration of the morphogen, another important parameter for tissue patterning is related to the duration of morphogen activity. 

Among the main morphogens that play a critical role in cell–cell signals in both development and disease, we found members of the  Wnt family, fibroblast growth factor (Fgf), hedgehog (Hh), transforming growth factor beta (TGFb), and retinoic acid (RA). 
Morphogen gradients incorporate a range of mechanisms including short-range signal activation, transcriptional/translational feedback, and temporal windows of target gene induction. 

!!! Do not confuse the term morphogen with morphogenesis!!!