196 THE EYES OF THE BLIND VERTEBRATES OF NORTH AMERICA. 



metres long. It is, however, not unusually developed in the earlier stages before 

 hatching, and shortly thereafter when the cessation of cell division, the most impor- 

 tant element of the stunted optic development, takes place. Besides this the tactile 

 organs of Chologaster, which possesses normal eyes, are very highly if not so elab- 

 orately developed as in Amblyopsis. I have experimentally determined by elimina- 

 ting the eyes altogether that the tactile organs in Chologaster papilliferus are amply 

 developed to enable the fish to live indefinitely without the use of its eyes. The same 

 must also be true of Chologaster agassizii, which lives permanently in caves. While 

 not impossible, it seems, therefore, very improbable that the tactile organs affect the 

 development of the eyes in Amblyopsis and not in Chologaster.* 



I know of no other organs in Amblyopsis whose development differs from that 

 of Chologaster in a degree sufficient to make it a successful contestant for a food- 

 supply in Amblyopsis and not in Chologaster. 



What has been said concerning organs whose presence might affect the develop- 

 ment of the eyes is equally true concerning organs whose absence might deprive the 

 eye of the necessary stimulus to reach normal development. I know of no organ, 

 either in Amblyopsis or Chologaster, whose absence in the one and presence in the 

 other might account for the difference in the degree of development reached by the 

 eyes in the two fishes. 



The conclusion is forced upon us by the above considerations that neither in the 

 environment nor in the fish itself is there a factor sufficient to account for the early 

 arrest in cell division, the retardation of the morphogenic processes, and the stopping 

 of the histogenic processes. We are therefore entirely justified in assuming that 

 the determining cause of the method of development lies in the cells themselves and 

 is inherited. The great development of the scleral cartilages beyond the needs of 

 the eye also tend to locate the formative or hereditary power in the cartilages them- 

 selves rather than in the stimuli to their development that they receive from their 

 contact with the developing eyes, for they develop entirely beyond the needs of these 

 eyes. 



The causes operating in ontogeny and phylogeny that have led to the limited 

 power of development and differentiation I have fully considered in my first paper 

 (Eigenmann, '99, p. 596) in a chapter which was also published in the Popular Science 

 Monthly. It was there concluded that the phylogenic degeneration, which is equiv- 

 alent to saying the limited power of development found in the cells entering into the 



* As an example bearing on this subject it may be permitted to call attention to the tactile apparatus of the 

 Siluridse, which is certainly in many instances more elaborate than that of Amblyopsis and yet the eyes are normal 

 though small. 



