Frederick William I, the King of Prussia, ____ that his greatest gift to the Russian people would have ____. This gift was the Amber room, which was given this name because several tons of amber were used to make it. The amber ____ had a beautiful yellow-brown colour like honey. The design of the room was ____ popular in those days. It was also a treasure ____, which took the country's artists about ten years to make.

胚胎的发育场所是（    ）