570 THE WONDER OF LIFE 



ought by rights to be endodermic, is formed by the ectoderm. 

 There are not a few cases of this sort, and it is plain that 

 regenerative growth does not necessarily follow the path 

 of embryonic development. The same sort of difficulty 

 arises in connection with the buds of Tunicates and 

 Polyzoa which reach the same general result as a fertilized 

 ovum, but by quite different paths. Perhaps we have 

 here a hint that we may create unnecessary difficulties by 

 making too much of the distinctiveness of the germinal 

 layers. 



Another much- discussed case must be cited. Calucci, 

 Wolff, and Muller have made the experiment of extracting 

 the lens of Triton, with the result that it was normally 

 regenerated. From what, however ? Not from remnants 

 of the lens, for there were none, but from the iris, with 

 which the lens (a product of the ectoderm in front of the 

 optic cup) has no genetic connection. That an iris should 

 be able to regenerate an iris would be consonant with the 

 general facts of regeneration, but it seems to be able to 

 re-make a lens, in whose original making it plays no part. 

 It may be pointed out that the posterior epithelium of the 

 iris is ectodermic like the lens, and furthermore that the 

 newt is an animal with great regenerative power in many 

 parts, and may be contrasted with an animal like the rabbit, 

 where regeneration of the lens does not occur unless some 

 portion of the lens be left. Perhaps, however, the case of 

 the newt's lens points to the conclusion that the residual 

 germinal capacity, localized here and there in animals, is 

 more general and less specific than is sometimes supposed, 

 and that what occurs in any particular instance is under 

 some sort of regulation, so that what is most needed tends 

 to be done. 



