On the 5th day after fertilisation, most of cells in the morula start to move to one side, creating a bunch of cells inside a sac of fluid (that the baby will float in). The individual cells (which all previously looked the same) now start to change (or 'differentiate') as they begin preparing to perform their individual functions, now destined to become the different parts of the baby's body (and the placenta).

(post com data alterada - publicado a 04/08/2006)