182 THE EYES OF THE BLIND VERTEBRATES OF NORTH AMERICA. 



longer be found (Fig. 37). In 7-millimetre larvae exactly half the eyes were without 

 a lens (Figs. 42, 43, 44), and in 9- to 10-millimetre larvae no trace of a lens could be 

 detected. The history of the lens is completed. Judging from this rapid and uni- 

 versal disappearance of the lens in the young I am inclined to the opinion that the 

 structure described in the adult eye as a lens is not a lens. 



The lens is the first organ to stop developing, the first to begin to degenerate, and 

 the first to disappear. 



X. THE HISTORY OF THE SCLERAL CARTILAGES. 



Attention was called in my first paper (Eigenmann, '99, p. 563) to the variation 

 of the scleral cartilages. A study of the development of the cartilages has enabled 

 me to detect perhaps a greater degree of uniformity of plan if not of structure in these 

 cartilages than I was able to make out from a study of the adult alone. It would 

 seem that there are normally two cartilaginous bars of variable shape developed, 

 nat. history 83 



One or both of them may be replaced by two or more smaller cartilages. One of the 

 cartilages is found over the distal face of the eye and the other on the posterior face 

 caudad of the optic nerve. The earliest stages at which cartilages were noticed were 

 9.5 to 10 millimetres (PI. XV, Figs. 47, 48, 49) long. In one fish 10 millimetres long 

 there were in the right eye about ten cartilage-cells, all directly over the pupil and 

 iris. In the left eye there were about twenty-two cells, all over the dorsal part of 

 the iris, none of them in front of the pupil. There were no traces in these eyes of 

 scleral cartilages elsewhere. The cartilage-cells were still for the most part isolated, 

 not bound together into a definite cartilage. 



In another fish 10 millimetres long the cells were definitely bound together into a 

 small cartilage in each eye, that of one side encroaching on the pupil, that of the other 

 side not. 



In a fish 25 millimetres long there were two cartilaginous masses in each eye. 

 One of these was over the distal face of the eye, the other over the caudal face of the 

 eye caudad of the exit of the optic nerve (PL XII, Fig. 4). The one over the 

 distal face curved ventro-caudad. 



In a fish 30 millimetres long the cartilages were confined to the caudal half of 

 the eye and were developed in such proportions that they encroached on the eye. 

 The development of these cartilages to such unexpected size indicates that these 

 cartilages are self-determining and not conditioned by the stimulus to growth by the 

 eye with which they are in contact. In the right eye of this fish there were two 



