. The development of the human body : a manual of human embryology. Embryology; Embryo, Non-Mammalian. i6o DEVELOPMENT OP THE VERTEBRAE The Development of the Vertebrae and Ribs.—The mesen- chyme formed from the sclerotome of each mesodermic somite grows inward toward the median line and forms a mass lying between the notochord and the myotome, separated from the similar mass in front and behind by some loose tissue in which lies an intersegmental artery. Toward the end of the third week of development the cells of the posterior portion of each sclero- tome condense to a tissue more compact th
. The development of the human body : a manual of human embryology. Embryology; Embryo, Non-Mammalian. i6o DEVELOPMENT OP THE VERTEBRAE The Development of the Vertebrae and Ribs.—The mesen- chyme formed from the sclerotome of each mesodermic somite grows inward toward the median line and forms a mass lying between the notochord and the myotome, separated from the similar mass in front and behind by some loose tissue in which lies an intersegmental artery. Toward the end of the third week of development the cells of the posterior portion of each sclero- tome condense to a tissue more compact than that of the anterior portion (Fig. 96), and a little later the two portions become separated by a cleft. At about the same time the posterior por- tion sends a process medially, to enclose the notochord by uniting with a corresponding process from the sclerotome of the other side, and it also sends a pro- longation dorsally between the myotome and the spinal cord to form the vertebral arch, and a third process laterally and ven- trally along the distal border of the myotome to form a costal process (Fig. 97). The looser tissue of the anterior half of the sclerotome also grows medially to sur- round the notochord, filling up the intervals between successive denser portions, and it forms too a membrane extending between successive vertebral arches. Later the tissue surrounding the noto- chord, which is derived from the anterior half of the sclerotome, associates itself with the posterior portion of the preceding sclerotome to form what will later be a vertebra, the tissue occupying and adjacent to the line of division between the anterior and posterior portions of the sclerotomes condensing to form intervertebral fibrocartilages. Consequently each vertebra is formed by parts. Fig. 97.—Transverse Section through the intervertebral plate of the First Cervical Vertebra of a Calf Embryo of mm. be1, Intervertebral plate; mi, fourth myotome; s, hypochordal bar; XI, spinal ac
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