THE ORIGIN OF NEW CELLS AND TISSUES 197 



('95) and by Barfurth ('91). If we examine first the results of 

 Fraisse's study of the tail of urodeles, which have bony vertebrae, we 

 find the following changes take place. The cut-surface is covered by 

 the skin bending over the exposed part, accompanied by a migration 

 of cells from the edge of the ectoderm. Only the unspecialized 

 cells leave the old ectoderm to wander out over the cut-surface ; 

 gland cells and sense cells are entirely absent from the new ectoderm. 

 These kinds of cells develop later out of the undifferentiated cells 

 over the new part. The development of new vertebrae does not fol- 

 low the embryonic method of development. In the embryo the 

 endodermal notochord is first laid down, and around this and the 

 nerve-cord mesodermal cells accumulate to form the skeletal tissue. 

 Later the notochord is largely obliterated, as the vertebrae develop, 

 pieces of it being left along the vertebral column. In the regeneration 

 of the tail of the adult animal, the remnants of the old notochord 

 (even if exposed by the cut) do not take any part in the formation of 

 new tissue. In fact, there is no notochord formed at all. From 

 the injured vertebrae, or at least from their covering of skeletal tis- 

 sue, cells are proliferated, out of which a cartilaginous tube develops, 

 enclosing the new nerve-cord, which is growing out from the cut- 

 end of the old cord. In this tube centres of deposition of calcareous 

 material are formed, and the new vertebrae are produced in this way. 

 The new nerve-cord develops from the cut-end of the old cord, and 

 more especially out of the cells of the lining epithelium of the canalis 

 centralis. The new muscles develop from cells that arise from the 

 old muscles. 



In the tadpole of the frog the regeneration of the tail takes 

 place essentially in the way just described for the adult urodele, 

 except that, there being only a notochord in the tail, only a notochord 

 is regenerated. According to Fraisse, the new notochord develops 

 from cells that arise from the sheath of the old notochord, and not 

 from the vacuolated cells of the notochord itself. The notochord 

 cells are, he states, derived from the endoderm of the embryo, 1 while 

 the sheath arises from the mesoderm ; hence the newly regenerated 

 notochord that arises from the sheath of the old one comes from a 

 different germ-layer. Exception may be taken to this statement, 

 because in the frog's embryo the notochord develops from tissue that 

 is at first perfectly continuous with the mesoderm, and, in fact, may 

 be called mesoderm ; also because it is probable, in the light of more 

 recent research, that both the notochord and its sheath have exactly 

 the same origin. 



1 This seems to be true for urodeles, but whether it is true for the anurans is rather a 

 question of definition, as I have pointed out in my book on The Development of the frog's 



