132 



BLIND VERTEBRATES AND THEIR EYES. 



will suggest the homology of the anterior cell mass in the latter case, with the pig- 

 ment cells always present between the retina and the irideal pigment layer in the 

 former species. This correspondence is further strengthened by the fact that 

 frequently the pigment in T. rosa over the front of the eye is in more than 

 one layer of cells. Since, however, I was unable to arrive at an entirely satis- 

 factory explanation of the origin of this pigment mass in Amblyopsis, it will not 

 help us much, should the two structures be homologous. 



Attention may be called here to the fact that both in Amblyopsis and in the pres- 

 ent species the lens — and therefore the lost pupil — are not situated at the distal 

 pole of the eye, but above this point, and that both in regard to the pupil and the 

 eye in general the location of the pigment masses in the two species is the same. 



The pigment is granular, ,iot prismatic. 



scl. c 



Fig. 48. Cross-sections through Right and Left Eye of an Individual 25 mm. long. Sections be pass through Lens. 

 Fig. a is a Composite from 3 Sections. Fig. b represents one Section, but the '* Lens " is from the Next Section. 



The lens is the only structure of the eye concerning which Kohl has not made 

 any mistake. 1 It is a small group of cells closely crowded together and about 

 10 or 12 fj. in diameter (figs. 48 a' and b, I). There are no signs of fibrilation or the 

 result of any other histogenic process; it appears as an aggregation of indifferent 

 cells. On its surface there are at times cells that are evidently of an epithelial 

 nature, being flattened so that their sections appear much longer than deep. It 

 lies at the upper outer face of the eye at the margin of the pigment mass described 

 in the last section. It is not covered by pigment or other retinal substance. 

 Kohl considered this condition a primary one. The lens, however, does not lie in 

 an incipient secondary optic cavity, the vitreal cavity, as Kohl supposed, but in 

 the remains of such a structure. Under the circumstances it is doubtful whether 

 the uncovered condition is primary. It seems more probable, considering the 

 condition in Amblyopsis, that the lens was inclosed by the closing of the pupil 

 over the eye, and that the present naked condition is the result of the subsequent 

 degeneration of the iris over it. That the latter is the phylogenetic origin of its 

 present condition there is no doubt. 



1 Considering the history of the lens in Amblyopsis, I am not sure now whether Kohl was or was not mistaken 

 about these cells. 



