198 RE GENERA TION 



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 

 vertebrae, but in the middle of a vertebra in some species the seventh 

 caudal. The vertebras are thicker at their ends than in the middle, 

 and are firmly held together by intervertebral cartilages. The cen- 

 tres of the caudal vertebras 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. 1 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 

 vertebrae, as does the new tail of the salamander, but, instead, a car- 

 tilaginous 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 sur- 

 face of ectoderm is found covering the end of the tail. The new ecto- 

 derm 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 noto- 

 chord, that are present in the vertebra, have nothing to do with the 

 new structure, nor does the new tube represent in any way a noto- 

 chord, 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 vertebrae. 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 sur- 

 rounding 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 



1 The attachments of the muscles may be the cause of the break in the middle of the 

 vertebrae, rather than between two vertebrae. 



