224 BLIND VERTEBRATES AND THEIR EYES. 



so, becoming even larger in those eyes which become distended. In the eyes which 

 contract, on the other hand, it becomes closed and the opening finally is entirely 

 obliterated. The irideal parts are then indicated by a layer of pigment much thicker 

 than elsewhere about the eye (plate 26, figs, e and f). This pigmentation over the 

 front of the eye is not unique in the Cuban blind fishes. It is well marked in 

 Typhlomolge and so striking in Troglichthys that Kohl in this species ruled the 

 irideal pigment entirely out of the eye. Where the iris joins the retina, in the ciliary 

 region, there are cells with elongated nuclei as usual. 



THE VITREOUS BODY. 



The vitreous space and its history differ greatly in different eyes. For instance, 

 in 125 a, right, it is very large, while on the other side it is almost entirely filled with 

 the enlarged hyaloid blood-vessels (plate 25, figs. F and g). The factors that con- 

 dition its structure in these two eyes also control its later history. In the eyes that 

 contract the space becomes rapidly reduced and finally becomes obliterated (plate 

 26, figs. A and f). In the eyes that become vesicular it remains, unless the vesicu- 

 lation is so pronounced that the eye becomes a hollow sphere with the pigment 

 layer forming the larger part of the circumference, and the retina is literally turned 

 out to form the front part of the sphere. In the latter case the vitreous space, or 

 body, naturally is turned entirely out of the eye. 



Between the vitreous body and the hyaloid membrane, or in the latter, pigment 

 cells are sometimes found in old individuals. 



THE RETINA. 



The pigment layer in the young 20 mm. long is a thin epithelial layer, well pig- 

 mented. In places where it is artificially separated from the rest of the retina 

 processes extend from the pigment layer down, and from the nucleated layer up, 

 indicating more than mechanical contact between the layers (plate 25, fig. g). 

 Such conditions argue that there is a beginning, at least, of the differentiation of 

 cones. The pigment layer never becomes more highly differentiated than in 

 these young. In eyes that become distended the amount of pigment being scat- 

 tered over a wider area is much less dense at any one point. (Compare right and 

 left eyes of 126, plate 26, figs. A and d.) 



Sometimes pigment cells, or simply accumulations of pigment, separate from 

 the layer and come to lie between the layer and the rest of the retina. These are 

 also found in Lucifuga (plate 26, fig. a). Whether this is a case of active migration 

 of pigment cells or simply a result of mechanical crowding, I am unable to say. 



The remaining layers of the retina may best be considered together. In young 

 20 mm. long (plate 25, figs, e, f, g) these consist of a ganglionic layer consisting of 

 several series of cells. A nuclear layer, also consisting of several series of cells, lies 

 immediately beneath the pigment layer. The nuclei are similar, there being little 

 differentiation. Between the nuclear layer and the ganglionic layer is a sharply 

 defined, broad, reticular layer. This is differentiated into a wider outer, a nar- 

 rower inner, and a very narrow, more densely staining intermediate layer. In 

 favorable sections stratifications can be made out in this entire layer. 



Mullerian fibers are seen, but I have not identified the nuclei belonging to them. 





