no THE GERM-PLASM 



foundation, the notochord, is never formed anew. The carti- 

 laginous sheath of the notochord has an important share in the 

 primary formation of the vertebral column, but it disappears 

 to a greater or less extent at a later stage. If it became pos- 

 sible for the vertebrae to undergo regeneration after a portion 

 of the tail had been lost without a renewal of the notochord 

 taking place, the result would be a useful abbreviation of the 

 process of regeneration. Such an abbreviation has occurred, 

 and everything supports the assumption that at an earlier stage 

 of phyletic developmetit the notochord was capable of undergoing 

 regeneration, and that it has only lost this capacity secondarily. 

 In the case of frog-tadpoles, the power has been retained of re- 

 generating the tail when it is cut off together with the notochord. 

 We must not assume that the notochord does not become 

 restored in other amphibians because it no longer persists in the 

 full-grown animal ; for it is entirely absent only in a few of 

 them {e.g., Salmandrina^, and the notochord of the larval sala- 

 mander cannot be regenerated any more than that of the adult. 

 Thus the capacity for regenerating the notochord has been lost 

 by most amphibians in the course of phylogeny. Such a process 

 of degeneration is certainly to be explained most easily by 

 assuming the existence of special regenerative determinants, 

 which may gradually disappear without in the least affecting 

 their embryogenetic partners. 



The necessity of this assumption is shown still more conclu- 

 sively in the case, for instance, of the restoration of the solid 

 axis of the tail in reptiles. The tail of a lizard quickly becomes 

 restored after it has been cut off, but its structure is then different 

 from that of the original tail ; for, according to the statements 

 of Leydig and Fraisse, the spinal cord and vertebral column 

 are not renewed. The former is, however, represented by an 

 epithelial tube, but gives off no nerves ; and the latter is replaced 

 by an unsegmented cartilaginous tube. As Fraisse points out, 

 this tube does not correspond to the regenerated notochord, but 

 is a new structure which is substituted for it. 



A phyletic development, tending essentially towards a sim- 

 plification of the parts, has taken place in this case as re- 

 gards the processes of regeneration. A gradual degeneration 

 has occurred, just as may take place in the tail or any other 

 organ of an animal in the course of phylogeny. The caudal 

 region of the vertebral column has undergone a reduction. 



