The Animal Organism and its Germ-Layers 57 



(c) Evidence from the Regeneration of the Lens of the 



Amphibian Eye 



The supremacy of the organism over its germ-layer is 

 shown in no way more strikingly than in facts brought to 

 light in some of the researches of late years on animal re- 

 generation. Perhaps the case at once the best established 

 and most discussed is that of the way the extirpated lens 

 of the eye is renewed in some amphibians. Referring to the 

 parts enumerated above as arising from each of the three 

 germ-layers in vertebrates, we note that the lens is assigned 

 to the ectoderm. To be a little more explicit, this member 

 originates from the outermost epithelial layer of the head 

 of the embryo. Hardly any point in vertebrate embryology 

 is easier to demonstrate than this. Experiments have 

 proved, however, that when a new lens is produced in a full 

 grown animal, to take the place of one that has been de- 

 stroyed, tliis does not arise as did the original from the 

 surface epithelium but from the edge of the iris, that is, 

 from a part of the eye itself. The discoverer, Collucci, did 

 not consider it to be out of accord with the germ-layer doc- 

 trine because the iris is the highly modified rim of the orig- 

 inal optic cup, which is derived from the cerebral vesicle, 

 which in its turn is derived from the ectoderm, so that in a 

 round-about way the iris is an ectodermal product. The 

 fact remains, nevertheless, that the mode of origination of 

 the new lens is so radically different from that of the old 

 that to regard it as sufficiently dealt with when attention 

 is called to the fact that it conforms in a way to the genn- 

 layer doctrine is an impressive illustration of the evil effects 

 of subserviency to a theory — of the inhibiting effect of such 

 a mental attitude upon interest and observation. Later 

 investigators, notably G. Wolff, A. Fischel, and W. N. 

 Lewis, have shown how much more there is to the phenomena 

 of development and regeneration of the vertebrate eye than 



