﻿2l2 BLIND VERTEBRATES AND THEIR EYES. 



the center. In these early stages the cartilage is usually in contact with the iris in 

 front, but diverges widely from the eye proximally and not infrequently extends 

 beyond the eye. The inevitable conclusion is reached by an examination of such 

 figures as scl. c. of plate i6, figure a; plate 17, figures A, b; plate 18, figure B, 

 that the sclera was built for an ontogenetically or phylogenetically much larger 

 eye than the largest found, and that the sclera has not been reduced at the same 

 ratio as the eye itself. There is here no possibility of an artificial shrinking causing 

 the space between the sclera and the eye, because this space is filled with undis- 

 turbed tissue, and the only indication of a shrinking is sometimes noticeable prox- 

 imal of the eye, between it and the fibrous part of the sclera. 



The ratio of the largest eye found in the young (76 a) to the eye suggested by 

 the sclera is about as 45 to 85 ; in the smallest eye among the young of 76 (/. c. 76 b), 

 it is about as 20 to 49. The eye, however, even if as large as suggested by the 

 scleral cartilage, would still be a very small eye, unless the scleral cartilage formed 

 but a rim over the front of the eye. 



The cartilage is only about one cell deep, except near the outer rim where it is 

 occasionally thickened. Over the back of the eye stretching from the proximal 

 edge of the scleral cartilage there extends a slack membrane very much thinner than 

 the cartilage and apparently continuous over the surface of the cartilage as an ex- 

 ceedingly thin membrane. Near the scleral cartilage this proximal membrane has 

 a definite outline which is at times lost toward the optic nerve, the membrane 

 becoming flocculent and its substance less readily distinguishable from the con- 

 nective tissue filling the socket. A similar membrane more uniform in outline 

 and consistency over the front of the eye represents the cornea. 



The scleral cartilages degenerate shortly after birth. In the eyes of recently born 

 individuals they differ from those in the eyes of the unborn by fitting close to the 

 bulb. They have apparently been drawn to the bulb and in this process lost their 

 symmetrical shield shape and are at times bent in acute angles, at other times their 

 free margins project considerably beyond the eye. In one case, an individual 

 (No. 203) 25 mm. long (scl.c. plate 19), the cartilage in shrinking to the eye was 

 thrown into a fold extending some distance from the eye. The pockets formed 

 between the layers of cartilage in this fold are filled with pigment apparently belong- 

 ing to the retina. This peculiarity is found in both eyes of this individual. The 

 cartilages in free living individuals are much more variable than in the unborn young, 

 and even in one individual only 28 mm. the cartilage of one eye has entirely dis- 

 appeared, while that in the other is a minute bar folded upon itself. In only a 

 single case, to be described shortly, were there any traces of cartilage in specimens 

 over 40 mm. long. The fibrous part of the sclera differs greatly in thickness in 

 different eyes of older fishes or even in the same eye. 



The greatest amount of difference between the sclera of the mother and the 

 unborn young described above (76) is undoubtedly found in the cartilage. In the 

 right eye of the mother there is no definite cartilage at all; there is a nodule of 

 substance at the lower margin of the iris that may be the remnant of the cartilage, 

 but otherwise there is nothing in this eye to indicate that there ever was any car- 

 tilage associated with it at any time. In the left eye of the same individual are two 

 nodules of cartilage, one tangent to the dorsal surface of the eye (plate 21, fig. a), 

 the other in a vertical section through the middle of the eye somewhat below the 



