THE EYES OF TROGLICHTHYS. 127 



the inner layer may have progressed farther in one eye than in the other, but there 

 is always a considerable space still left between the inner and the outer layers of 

 the primitive eye vesicle. The elements of the inner layer, the ganglionic cells, 

 he found to send their processes directly inward. They must have gradually re- 

 volved, since in the normal eye the nerve processes are directed outward. Some 

 of the fibers cross each other on their way to the outlet for the nerve. Not all of 

 the invaginated cells send processes. Among those that do there are smaller, round 

 cells without a trace of fibers. From these the rest of the nervous parts of the 

 retina, including of course other ganglionic cells, would probably have arisen. 

 The outer layer of the secondary eye vesicle is also single-layered. The cells are 

 elongate, with oval nuclei, and without a definite arrangement. They are con- 

 nected with the few cells of the optic stalk that still remain. Connective tissue 

 cells are found in the nervus opticus. They are probably mechanically active in de- 

 generation by separating the elements. He found no sheath to the optic nerve, 

 as described by Wyman. The lens he found to be a spherical cell heap o.oi mm. 

 in diameter in the distal pole of the eye. It lies just within the sclera and the cup 

 of invagination. The sclera is made up of several layers of very fine fibrillse. 

 Nuclei are not found in it, but nuclei are found on its outer surface. No vessels 

 are found in the choroid, which consists of connective tissue cells more numerous 

 on the dorsal than on the ventral surface. The Typliliclithys eye is "absolut 

 pigmentlos." The surrounding tissues are rich in pigment, which, however, is not 

 related to the eye. There are pigment masses found here and there, but especially 

 between the bulb and the cartilaginous capsule. 



It is hard to arrive at the proper explanation of the structure of this highly 

 degenerate eye even with an abundance of material, and it is probably not to be 

 wondered at that Kohl in the work outlined above did not see the eye muscles, 

 mistook the sclera for suborbitals, parts of the retina for the choroid, interpreted 

 the pigmented epithelium of the eye as an extra optic pigment mass, mistook the 

 inner reticular layer for the primary optic cavity, the nuclear layers for the pig- 

 ment epithelium, etc., and arrived at a thoroughly erroneous idea of the general 

 structure of the eye and based his theories on the degeneration of eyes in general 

 on his conception of the structure of this eye. The invaginating cells of the 

 primary optic vesicle are supposed to have been directly converted into the gang- 

 lionic cells, which are usually among the very last products of the histogenesis of 

 the retina. 1 



By supposing that the eye was arrested at the beginning of the invagination, 

 and that the invaginating cells rotated on their axes and were converted directly 

 into ganglionic cells, Kohl derived the nucleated layers from the outer pigment- 

 producing layer of the primary vesicle, at the same time ruling the pigment layer 

 out of the eye. 



The eye is very small and situated so deep that it is impossible to see it from 

 the surface (fig. 44 a). In the upper half of a head cleared in xylol it is just evi- 

 dent to the naked eye as a minute black dot (figs. 44$, c). As in Typhlichthys 

 and in Amblyopsis, it is surrounded by a fat-mass filling the orbit. It is not at all 



1 The mistakes of Kohl, especially as far as they are the result of criticising work done on Amblyopsis while 

 he was working on another species, seem to me to point a moral. A certain species must not be too readily taken 

 as an exponent of a family, order, or class, and a knowledge of related species and geographical distribution is not 

 altogether to be neglected. 



