AMPHIOXUS

The extant adult amphioxus, or lancelet, is considered to resemble an ancient ancestor of the vertebrates (see Plate 1-1). It is a fishlike animal, about 2 inches long, that has the basic body plan of the early human embryo. The central nervous system consists of a nerve cord resembling the portion of the human embryonic neural tube that becomes the spinal cord. The digestive, respiratory, excretory, and circulatory systems of the amphioxus also closely resemble those of the early human embryo. As in the early human embryo, the skeleton of the amphioxus consists of a notochord, a slender rod of turgid cells that runs the length of the body directly beneath the nerve cord, or neural tube. The muscular system of the amphioxus consists of individual muscle segments on each side of the body, known as myotomes or myomeres, which are similar in appearance to the myotomes of the early human embryo. The nerve cord of the amphioxus gives off a pair of nerves to each myotome, and the striated muscle fibers of the myotomes contract to produce the lateral bending movements of swimming.

AXIAL SKELETON

The axial skeleton includes the vertebrae, ribs, sternum, and skull. The first structure of the future axial skeleton to form is the notochord (see Plate 1-1). It appears in the midline of the embryonic disc at 15 days of development as a cord of cells budding off from a mass of ectoderm known as Hensen’s node. The notochordal cells become temporarily intercalated in the endoderm, which forms the roof of the yolk sac. After separating from the endoderm, the notochord becomes a slender rod of cells running the length of the embryo between the neural tube and the developing gut.

The dorsal mesoderm on either side of the notochord becomes thickened and arranged into 42 to 44 pairs of cell masses known as somites (4 occipital, 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 8 to 10 coccygeal) between the 19th and 32nd day of development. The formation of these primitive segments, or somites, reflects the serial repetition of homologous parts known as metamerism, which is retained in many adult prevertebrates. The vertebrate embryo is fundamentally metameric, even though much of its segmentation is lost as development proceeds to the adult form. The first significant change in the somite of the human embryo is the formation of a cluster of mesenchymal cells, the sclerotome, on the ventromedial border of the somite (see Plate 1-2). The sclerotomal cells migrate from the somites and become aggregated about the notochord to ultimately give rise to the vertebral column and ribs (see Plate 1-3).

VERTEBRAL COLUMN AND RIBS

During the fourth week of development, a clustering of sclerotomal cells derived from two adjacent somites on either side of the notochord becomes the primordium of the body, or centrum, of a vertebra. Soon after the body takes shape, paired concentrations of mesenchymal cells extend dorsally and laterally from the body to form the primordia of the neural arches and the costal processes. The costal process becomes a rib that articulates with the body and transverse process of the neural arch of the thoracic vertebrae (see Plate 1-4). The costal process becomes the anterior part of the transverse foramen of the cervical vertebrae, the transverse process of the lumbar vertebrae, and the lateral part of the sacrum. Occasionally, the costal process of the seventh cervical or the first lumbar vertebra becomes a supernumerary rib. Failure of fusion of the neural folds results in various types of spina bifida.