Any construction has to have a timing plan. Parts have to be set up to put in place in an orderly fashion. In a building you cannot sheetrock before wires and pipes in the wall are in place.

The same is true in the embryo, but in the beginning all one sees is a ball of blast cells. Eventual areas of the potential 'body' must be identified, placed properly and then development started. Here is the initial clockwork process:

http://www.sciencedaily.com/releases/2011/10/111013153943.htm