DEVELOPMENT OF THE EYE OF AMBLYOPSIS. 167 



that phylogenetically younger structures lose their power of cell division earlier 

 than phylogenetically older ones. 



The retardation of the morphogenic processes, cell arrangement, movement, 

 union and separation, etc., is conspicuous in the delay of the closing of the choroid 

 fissure and all that this implies. There is no conspicuous stopping of this process 

 except in the occasional failure of the choroid fissure to close at all. 



Histogenic processes are also distinctly retarded, and in conspicuous instances 

 suffer an entire stoppage. While the eyes of 3-rara. specimens of Cymatogaster 

 or Carassius and Amblyopsis are nearly alike, in the former two the tissue differ- 

 entiation has progressed vastly farther by the time the fishes have reached a length 

 of 10 mm. Histogenesis is carried surprisingly far in many degenerate eyes. In 

 Rhine ura, for instance, the layers of the retina are differentiated far beyond the 

 requirements of the case. In Amblyopsis the process, as far as it can be made out 

 with the methods available, falls short of the normal development. 1 The cells of 

 the lens never lose their embryonic characters ; they are never transformed into lens 

 fibers. 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 ontogenetic stages. 

 The question whether these are also phylogenetically young may be passed over. 



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 developed than has 

 been noted in normal eyes, the point to which cell division, cell arrangement, and 

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

 leading to the changed development are of approximately equal value in different 

 specimens from the same locality. 



CAUSES OF RETARDATION AND CESSATION IN THE DEVELOPMENT OF THE EYE. 



The retardation and arrest in the ontogenetic development of the eye of Amblyop- 

 sis 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 predeter- 

 mined in the egg cell from which the eye is ultimately derived. The conditioning 

 factor, if it lie outside 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 exer- 

 cised by some other part of the body. Unless we assume that the eye of Ambly- 

 opsis 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 retarda- 

 tion and cessation. 



Although, in discussing the phylogenetic degeneration of the eyes of cold- 

 blooded vertebrates in general, I have insisted that cross-country conclusions must 

 be guarded against, I then saw no objection, and now see none, to considering the 

 different members of the Amblyopsidse as homogeneous material within the bounds 

 of which we may expect similar causes to effect similar results. The different stages 



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

 diameter falls short of 0.2 mm. can readily be imagined. 



