Answer to the Parasite Case of the Week 638: Not a human parasite; seed.
This is a small seed that likely became embedded in the nail. Botanist Dr. Mary Parker commented that "I would rule out a grass/cereal seed, as there is clearly an embryo (radicle and cotyledon) embedded in starchy endosperm, the whole surrounded by an epidermis and cuticle. The central vascular strand can be seen in the radicle, with pro-vascular cells in the cotyledon. It must be a small seed but in the absence of a scale bar, it is difficult to say more." I learned from Dr. Cynthia Magro who donated this case that the patient has onychomycosis, so it is understandable how a small seed could have become trapped within a thick, roughened nail.
Here are some of the morphologic details for those of you who are interested!