Vertebrates 



691 



the origin of the regenerating lens from the 

 iris epithelium instead of the corneal epi- 

 thelium — a distinct change from its ontog- 

 eny. This viewpoint was a rebellion against 

 the Haeckel dominance and was primarily 

 directed against Darwinism as championed 

 by Haeckel. Wolff's findings were imme- 

 diately in the spotlight; how much so is 

 shown by the attachment of his name in- 

 stead of Colucci's to this type of regeneration. 

 Since the conclusions were advanced against 

 Darwinism, the cudgels of the scientists were 

 immediately brought to the attack. The re- 

 sults were immediately attacked and experi- 

 ments repeated. Everyone was sceptical, but 

 the repetition of the work and its confirma- 

 tion by E. Miiller (1896) and others, with 

 the discovery by Emery of Colucci's work, 

 made Fischel's ('00) later confirmation a 

 redundancy. 



Wolff showed that the dorsal border of 

 the iris was the formative area in lens re- 

 generation. From his viewpoint this was the 

 most advantageous place for it to form, for 

 as it increased in size it practically fell into 

 place in normal location. He followed also 

 the extrusion of pigment which occurs at 

 the border of the iris during lens formation. 

 Wolff's observation was an important one. 

 He was opposed by Weismann, who took 

 the stand that regeneration was a power 

 acquired by natural selection. Wolff attempt- 

 ed to show (1) that the reaction was pur- 

 poseful, and (2) that purposefulness is a 

 common property, primitive in nature, which 

 can be explained neither by heredity nor 

 by natiural selection. 



Fischel took the view that the process had 

 no purpose, citing the many anomalies which 

 occurred in the nature of imperfect lenses 

 or double formations. To him the process 

 was a mechanical one. The limitation of 

 regeneration to the dorsal border of the iris 

 was explained by gravity. He invoked tur- 

 gor acting from the region of the vitreous 

 as inhibiting regeneration. He did prevent 

 regeneration in one series of experiments by 

 transplanting cornea into the eye, thus in- 

 creasing the internal pressure. When, how- 

 ever, he introduced foreign bodies such as 

 bread pills and potato, his results were un- 

 successful for lens inhibition, but were 

 highly successful in attracting Wolff's rid- 

 icule. The controversy grew from the acid- 

 ulous to the philippic, for Wolff was clever 

 in both argument and experiment. He cut 

 the spinal cord of his animals to inactivate 

 them, then placed them on their backs until 

 regeneration had occurred. Regeneration oc- 



curred as before from the original dorsal 

 border of the iris and Fischel's idea of the 

 action of gravity was exploded. In the course 

 of these experiments he found that Triturus 

 completed the process faster than Sala- 

 mandra; the axolotl was very slow in re- 

 generation if it occurred at all. 



Wachs ('14) recalled the problems of the 

 eye. His paper gave an excellent review of 

 the background of the work and laid out 

 clearly many of the problems which could 

 be approached. He worked out the variations 

 of the process in different species and the 

 details of factors, such as the age of the 

 animal at the time of regeneration, the de- 

 gree of differentiation at the time of con- 

 striction from the iris, and its relative size. 

 He gave proof to Fischel's contention that 

 turgor was a controlling factor in regenera- 

 tion. He supported and extended Wolff's 

 findings. The lens regeneration occurs with- 

 out injury to the iris, since it occurs if the 

 lens is removed through the roof of the 

 mouth instead of through the cornea. When 

 the iris is injured, regeneration is slowed 

 and the lens may be less perfectly formed 

 than when the iris is intact. Injury is not 

 necessarily a factor, since the lens may be 

 removed and replaced after which no regen- 

 eration takes place. If a small lens from a 

 younger animal is placed in a host, regen- 

 eration is inhibited. The small lens grows 

 more rapidly (younger tissue) until it 

 reaches the size appropriate for the host eye. 



Wolff used grafting experiments to study 

 the effects of the lens upon regeneration. 

 These are partly mechanical and partly 

 chemical, reacting to a stimulating chemical 

 effect from the retina. If the lens is pushed 

 posteriorly into the vitreous, there is an 

 initial reaction all around the border of the 

 iris as though regeneration were about to 

 occur; however, it never is completed. If 

 the lens is removed and minced lens placed 

 in the eyeball, no regeneration occurs until 

 the minced lens is resorbed. The first experi- 

 ment might be due to a mechanical effect, 

 but the second seems to have removed this 

 completely and indicates the chemical con- 

 trol of the reaction. If the eye receives the 

 grafting of an extra dorsal border of the 

 iris, it will not form a lens until the host 

 lens is removed, but when this is done two 

 lenses regenerate, one from the grafted iris 

 and one from the normal one. If the dorsal 

 border of the iris is transplanted into the 

 vitreous humor after lens removal, it forms 

 a lens more quickly than it would in its 

 original location. Lenses are not formed in 



