It is known that the tail of lizards breaks off generally at a definite region near the base, and that the break does not occur between the vertebræ, but in the middle of a vertebra--in some species the seventh caudal. The vertebræ are thicker at their ends than in the middle, and are firmly held together by intervertebral cartilages. The centres of the caudal vertebræ are the weakest links in the chain, or at least the place at which the vertebral column is most easily broken in response to the contraction of the tail-muscles.[94] Fraisse and others speak of this arrangement as an adaptation for breaking off the tail. The new tail that regenerates does not contain a new series of vertebræ, as does the new tail of the salamander, but, instead, a cartilaginous tube that is attached to the half of the broken seventh caudal vertebra. The regeneration of the new tissues of the tail of the lizard takes place as follows: A scab forms over the cut-surface, composed in part of clotted blood, in part of broken-down tissues from the injured cells. In the course of a week the necrotic tissue falls off, and a smooth surface of ectoderm is found covering the end of the tail. The new ectoderm appears to come from the old, but its method of development has not been studied. The deeper layer of the skin of the lizard is composed of mesodermal connective tissue, and in the new part this layer arises from the connective tissue of the old part. The tissue that forms the cartilaginous tube of the new tail develops from the skeletal tissue of the broken vertebra. The remnants of the old notochord that are present in the vertebra, have nothing to do with the new structure, nor does the new tube represent in any way a notochord, but it appears to be a structure _sui generis_. In later stages, osseous plates may be formed in the cartilage, but these are too irregular to be compared to vertebræ. A tube grows out from the cut-end of the nerve-cord, which in some forms, as Fraisse shows, is only an extension of the lining epithelium of the nerve-cord. In other forms it is possible that other cells of the old cord may also grow backward, divide, and produce new cells. The fine thread that is formed in this way does not send out any nerve fibres into the surrounding parts. In _Anguis fragilis_, however, a few ganglion cells are present in the new cord. It is probable, Fraisse states, that while the new tube is morphologically a nerve-cord, yet physiologically it is not functional in any of the reptiles. The new muscles come from the old ones. Fraisse thinks that the new muscle fibres come from the so-called “spindle fibres” that split off from the primitive muscle bundles. These fibres, Fraisse believes, originate normally during the process of physiological regeneration of the muscles, and also after injury to the muscles. From these spindle cells the new muscle fibres develop in the same way as the muscle cells of the embryo.