Kindo: Dude... the embryo is the fetus. You're missing another item, most likely the arachas eyes - they can be bought from Cedric, or dropped by the Arachas you fight later on in the chapter.

As for the embryo, yes, they do drop from Endregas, quite frequently in fact. These enemies don't disappear from the forest once you've killed the queens, and can be found here and there. A good place to start looking is around those eggs you destroyed, or around the altar of Veyopatis.