Final answer:
The coccygeal portion of the spine, also known as the tailbone, typically consists of 3-4 vertebrae that fuse into one bone in adults.
Step-by-step explanation:
The coccygeal portion of the spine, which is commonly known as the tailbone, is formed by the fusion of four very small coccygeal vertebrae. In adults, the coccyx typically comprises these 3-4 vertebrae that fuse into one. However, the number can sometimes vary from person to person with some having only three coccygeal vertebrae. This fusion of vertebrae starts around the age of twenty and is usually completed by middle age.