By the end of the fifth week, the embryo has doubled in size and has grown a tail-like structure that becomes the coccyx (lowermost tip of the backbone).
Caudal vertebrae fused into a urostyle or coccyx.
In accordance with the saltatorial habits of the members of this order, the vertebrae, which number from 40 to 60 in the Caudata, to upwards of in the Apoda, have become reduced to Io as the normal number, viz., eight praecaudal, one sacral and an elongate coccyx or urostyle, formed by coalescence of at least two vertebrae.
The coccyx and the sacro-sciatic ligaments, together with the muscles attached to them, have been removed.
In some genera this coccyx is fused with the ninth vertebra, and contributes to the FIG.