Most likely the egg sac of a Ray Spider (Family: Theridiosomatidae, Genus: Theridiosoma, and probably Theridiosoma gemmosum). According to Bug Guide, there are only two species in North America. I first found one of these egg sacs a few years ago and always hoped to find another. A couple of days ago, by sheer chance I saw another one, this time on wild parsnip in Gatineau Park. The sac is about 1/4 inch or less in diameter.