Put in the broadest terms, the events taking place during the first four weeks of development following fertilisation lay the foundations for the development of the structures of the body (i.e. organ systems). Early embryonic development encompasses some key developmental events and processes that result in the production of sufficient cells, of the right type in the right place to ensure that structural development of the organs can proceed. These events and processes are linked and inter-dependent and can be conceptualised as a cascade of events where a given event primes the succeeding event.