RELATIVE STABILITY OF LIVING MATTER 337 



gives rise to muscle, nerve to nerve, bone to bone, and each part- 

 to its own kind by the simple method of growth extension. 



This, however, is the simplest method of regeneration, and 

 more complex methods are necessary in extreme regeneration, 

 where the entire supply of certain tissues is cut away, and the 

 new growth must arise from different tissue or not at all. 



Examples of the second class are less common, but not rare. 

 Where an entire limb is gone regeneration must proceed upon a 

 plan different from the one it would follow were only a single 

 tissue involved, like a wounded muscle. The first new growth 

 that arises must in some way be endowed with the power of pro- 

 ducing not one, but many, different kinds of tissues. Gotte, as 

 quoted by Morgan, has studied both the embryonic development 

 and the regeneration of the limb of triton, especially in regard to 

 the origin of the new bones. He found that the skeleton develops 

 in much the same way in the embryonic limb and in the regener- 

 ating limb, and the process in the latter may be said to repeat that 

 in the former." If the larva is young, the new growth differs but 

 little from the old, but if the bones had become ossified, the 

 difference between the two is much more marked. 1 Curiously 

 enough the salamander regenerates the tail completely, bones 

 and all, but the regenerated tail of a lizard contains not a new 

 series of bones, but a cartilaginous tube which is attached to the 

 broken seventh caudal vertebra. 2 



An example of the third case, in which tissue is regenerated 

 from a source different from that of embryonic development, is 

 shown in the reproduction of the lens of the eye of the salamander 

 or of triton. In this case, if the lens be removed a new one is re- 

 generated from the upper edge of the iris, a part of the body from 

 which the lens of the eye never develops normally in the em- 

 bryo of this or of any other vertebrate. In the embryo the lens 

 develops from the ectoderm at the side of the head, having no 

 connection with the iris. 



The regeneration from like tissue is no more difficult of com- 

 prehension than is ordinary growth, and that which repeats the 



1 Morgan, Regeneration, pp. 200-201. 



2 Ibid. p. 198. The lizard's tail does not break between the two vertebras, which 

 are strongly joined, but in the middle of the vertebra, which is relatively weak. 



