Photographic Dictionary lists the yolk of an egg as the only body part the begins with "y." There are no parts of the human anatomy that start with the letter "y."
Yolk features a yellow color and is the part of an egg that feeds an embryo as it develops. In non-egg-laying animals, the yolk is a part of the egg cell. Yolk is also called deutoplasm and makes up around 43 percent of the protein the embryo receives while in the womb. They accumulate during oogenesis.
Other organisms, such as algae, produce resources resembling the yolk in their female gametes.