THE EYES OF THE BLIND VERTEBRATES OF NORTH AMERICA. 193 



mens of Cymatogaster or Carassius and Amblyopsis are nearly alike, in the former 

 two the tissue differentiation has progressed vastly farther by the time the fishes have 

 reached a length of 10 miUimetres. Histogenesis is carried surprisingly far in many 

 degenerate eyes. In Rhineura, for instance (Eigenmann, :02), the layers of the retina 

 are differentiated far beyond the requirements of the case. In Amblyopsis the proc- 

 ess, as far as it can be made out with the methods available, falls short of the normal 

 development.* The cells of the lens never lose their embryonic characters; they are 

 never transformed into lens-fibres. Cones are rarely if ever developed in the retina, 

 and an outer reticular layer never. In normal development the cones and the outer 

 reticular layers are the last to differentiate, so that we have certainly a cutting off of 

 late ontogenic stages. The question whether these are also phylogenically young 

 may be passed over. 



4. Conclusion. — The total evidence from the three processes is that none of them 

 proceed with the push and rapidity found in normal structures, and though they are 

 normal they grow weaker with development and frequently give out altogether. 

 But with all this lack of vigor, while there is more variation in each structure devel- 

 oped than has been noted in normal eyes, the point to which cell division, cell arrange- 

 ment, and histogenesis are carried, in different individuals, is about the same. The 

 causes leading to the changed development are of approximately equal value in differ- 

 ent specimens from the same locality. 



XVI. CAUSES OF RETARDATION AND CESSATION IN THE 

 DEVELOPMENT OF THE EYE. 



The retardation and arrest in the development of the eye of Amblyopsis may 

 be due to one of several possible causes. They are either conditioned by something 

 outside the cells composing the eye, or they are inherent or predetermined in the egg- 

 cell from which the eye is ultimately derived. The conditioning factor, if it lies out- 

 side the eye, may be a peculiarity in the physical and chemical environment in which 

 the fish lives, or a lack of stimulation or an inhibition exercised by some other part 

 of the body. Unless we assume that the eye of Amblyopsis has reacted and does 

 now react differently to the physical and chemical environment from that of some of 

 the relatives of Amblyopsis, physical and chemical factors may readily be eliminated 

 as contributing directly to the retardation and cessation. 



Although, in discussing the phylogenic degeneration of the eyes of cold-blooded 



* The difficulties, for instance, of differentiating with Golgi methods the bipolar cells of an eye whose total 

 diameter falls short of .2 millimetre can readily be imagined. 



