For an example of this, we can use the arm and the leg. There is a pattern to the arm. Both the arm and leg consists of a number of different cell types including bone, muscle, nerves, blood vessels and skin. It is pattern formation that organizes the different cells to form an arm instead of a leg. Pattern formation is the process that enables the cells to know what they should become, and when and where they should do it.
Pattern evolves as the organism grows.
Pattern formation is not independent of the other 4 developmental processes First steps in pattern formation during development is the establishment of the body axes and germ layers. Today we discussed how the fertilized egg undergoes cleavage to become a multicellular embryo. Shortly after this stage the embryo begins to change shape and three different cell layers with different characteristics arise. These cell layers are called the germ layers for they will give rise to all the embryonic tissues that we see later in development. Later during development, as these cells differentiate an organized spatial pattern will emerge - such as the arm.