An emerging theme in developmental biology is the interplay of genetic and generic (i.e. physical) mechanisms[2, 15]. As the embryo grows its cells progressively organize into tissues with vastly differing physical properties extending from liquid (e.g. blood) to solid (e.g. bone). In more biological terms, different tissues are the consequence of cell differentiation. It is the particular differentiated state that gives rise to the specific physical-material properties of the cell. On the other hand the emerging physical characteristics feed back into development and restrict its outcomes[2].The embryonic state is particularly convenient for the study of this interplay because at this stage the organism is in a state of constant change involving active cell movement, it has not yet rigidified into more or less permanent structures and thus the effects of physical processes are more conspicuous.