Embryogenesis, the formation of a functional organism, containing 40 trillion cells that all arise from one single cell depends on three key elements: cell division, to make more cells, cell differentiation, to make different kinds of specialised cells and cell rearrangements, allowing cells to organise in specialised tissues. Active cell migration is a key mechanism of cell rearrangement. It is unknown when during mammalian embryogenesis cells gain the ability to move and if this happens before or after cells become specialised. I am investigating at what stage during embryogenesis cells gain the ability to migrate and how this migration is achieved mechanically and which molecules are key to this process. Understanding the mechanical changes embryonic stem cells (ESCs) undergo during early embryogenesis is essential to understand how a highly structured, well functioning organism is formed.