At about 41 days following ovulation, the embryo has doubled in size and is about 1/2 inch in length. Rays that will become the fingers and thumb can be seen. The lower jaw arch is becoming prominent, as well as the hyoid arch, a U shaped bone in the throat that allows for the production of a wide range of sounds that animals cannot produce. The grooves of the nose are distinct and there is a slight indication of a spinal curvature. Photo by Bradley R. Smith © 2009.