158 BLIND VERTEBRATES AND THEIR EYES. 



THE HISTORY OF THE LENS. 



The lens begins to develop when the embryo is about 2.5 mm. long (fig. 59 b). 

 It forms as a thickening of the skin where the optic vesicle is in contact with it. 

 It is still connected with the skin when the embryo has reached a length of 4.5 mm. 

 (Compare figs. 59 b, 59 c, 60 d, 60 e, 63 c with figs. 60 a, 60 b, the latter repre- 

 senting the development of a normal lens.) The history of the lens after this stage 

 is somewhat uncertain. It is well established that the cells composing it never 

 lose their embryonic condition, that they are never differentiated into fibers. In 

 many eyes, certainly in all in which a lens could be detected in later stages, the 

 lens becomes separated from the skin (fig. 60 e). The separation is completed 

 when the larva has reached a length of 5 mm. (fig. 62 a). From this stage on, 

 the lens begins to be resorbed ; in some 6-mm. larvae it could no longer be found 

 (fig. 62 b). In 7-mm. larvae exactly half the eyes were without a lens (figs. 63 b, 

 c, d), and in 9 to 10-mm. larvae no trace of a lens could be detected. The his- 

 tory of the lens is completed. Judging from this rapid and universal disappear- 

 ance 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. 



THE HISTORY OF THE SCLERAL CARTILACES. 



Attention was called 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 carti- 

 <20o° lages than I was able to make out from a study of the 



adult alone. It would seem that there are normally two 

 {c\ cartilaginous bars of variable shape developed. One or 



both of them may be replaced by two or more smaller 

 cartilages. One of the cartilages is found over the distal 

 u) Eyfof same a Fi : sh as that face of the eye and the other on the posterior face caudad 



from which fig. 63 g was - . . _, . . ... . 



taken. of the optic nerve. I he earliest stages at which carti- 



(6) Scleral Cartilage of Left r . ° 



Eye of Another Fish of lages were noticed were 9.5 to 10 mm. (figs. 63 g, 64 a, 0) 

 long. In one fish 10 mm. long there were in the right 

 eye about 10 cartilage cells, all directly over the pupil and iris. In the left eye 

 there were about 22 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 mm. 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 mm. 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 (plate 10, fig. b). The one over the distal 

 face curved ventro-caudad. 



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

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



