Bones of the human hand and wrist
At birth, the human infant typically has 270 bones, and many bones fuse together to reach a total of 207 bones in adulthood. However, some bones aren’t even formed yet in babies, when they’re born.
Included among those bones is the eight carpals, or wrist-bones. They begin to ossify (solidify) around 2 months of age, and continue to form through the beginning of puberty.
The last wrist bone to form (the pisiform) is a sesamoid bone, which forms part of the the ulnar border (near the pinky finger as opposed to the thumb) of the wrist.
Being a sesamoid bone means that it is a bone embedded within a tendon or muscle, and most often formed in response to strain. Unlike the other wrist bones, it doesn’t contribute to the rotation of the wrist, and is not an anchor for any major tendons or muscles.
Atlas and Text-Book of Human Anatomy. Dr. Johannes Sobotta, 1914.