Nancy, my first thought was that it is a white form of Allium cernuum, my standard-colored pinkish ones are blooming now.  But there don’t seem to be as many flowers as I would have expected.  See:

http://www.illinoiswildflowers.info/prairie/plantx/nod_onionx.htm

So I looked around for other nodding white varieties.  I found a few like A triquetrum and A paradoxum var normale, but their flowers are more bell-like and they don’t have the “exserted stamens, and an ovary with a style.  “ as A cernuum does. AND as do the flowers in your photo.

So, unless I come across something else or if someone else has something to contribute, it does seem like some version of Allium cernuum.

Kitty