That's a really good link you posted, Grapus. Although, if it were me, I would put unripe seed pod stems in water, if they couldn't finish ripening outside, rather than hanging them upside-down to dry.
Regarding embryos, these are some pic of the "lines" (embryos) withing the seeds. They are not always in the middle, I'd say 90-95%, though. Sometimes they hide. I always plant the seed if it looks fairly good, even if I don't see an embryo. "Plump" is a relative term. The outer portion is usually like tissue paper, or thinner. The inner portion is about paper thickness, or thicker. This would be a plump seed.
Most lily seeds look similar to these:
(they are lighted from behind)
But there can be variations:
