ABSTRACT
The development of the axial skeleton proceeds in a cranio-caudal direction, except for the atlas, which shows an aberrant development, and the first two vertebrae, which are retarded in development with respect to the following vertebrae. The vertebrae develop out of the neural arches, the perichordal tube and the hypochord. The future neural arches appear as condensations of mesenchyme opposite the myocommata. The atlas arches appear as condensations of mesenchymal cells posterior to the occipital arches and in front of the second trunk myotomes at stage 47. Perichondral ossification appears on the outer and inner surfaces of the neural arches at stage 54. Degeneration of the ventral cartilage formation in the perichordal tube starts anteriorly at stage 63. At stage 66 the m. longissimus dorsi takes origin from the urostyle and inserts onto the prootic-occipital complex.
