186 THE EYES OP THE BLIND VERTEBRATES OF NORTH AMERICA. 



palingenic processes operating on so deficient material give rise to conditions that are 

 not palingenic. In the closing of the choroid fissure of the normal eye the thing of 

 chief concern is the union of the infolded margins of the optic cup from the margin 

 of the pupil to the point of exit of the optic nerve and the closing in of the retina around 

 the optic nerve at its exit from the eye. In Amblyopsis the former process has 

 become insignificant, and the latter the prominent process. This is further compli- 

 cated by the fact that the vitreous cavity has ontogenically disappeared nearly as 

 much as phylogenically, so that, while the processes of changing the optic cup into 

 the eye are palingenic, the material operated upon being quite different from that 

 normally obtaining in fish embryos, the resulting stages of the eye are not palingenic. 



The choroid fissure, which is distally a distinct slit leading into what remains of 

 the optic cavity, becomes proximally a groove in a solid mass of cells. The closing 

 of this groove takes place at various times, or it may remain permanently open. This 

 condition has undoubtedly been brought about by a contraction of the area of the 

 retina and the consequent heaping up of cells, either concomitantly with, or as the 

 result of, the obliteration of the optic cavity. The funnel-shaped mass of cells in the 

 centre of the Amblyopsis eye is thus the result of the phylogenic rather than the onto- 

 genic disappearance of the optic cavity. 



I must confess that an easier way of explaining the developmental stages would 

 be reached by assuming that the central mass of cells, through which the optic nerve 

 passes, is not really ganglionic — that only the distal cells of the mass are ganglionic — 

 and that the proximal ones are the homologues of the cells found at the point of 

 entrance into the eye of Chologaster (PL XV, Fig. 50). This would imply that a 

 cavity has not disappeared from the centre of these cells (because there never was 

 one), and that the entire vitreous cavity has been reduced to that now found in the 

 embryo, and that no part of the cavity has disappeared in toto. This interpretation 

 is especially suggested by Figure 38. This would account for the fact that the optic 

 nerve does not form a central strand through the funnel of ganglionic cells, but passes 

 through it in several strands as it does through the mass of cells at the entrance of 

 the optic nerve (Fig. 50). The objection is that it would not account for the position 

 of the exit of the optic nerve, which should, according to this view, be at the proximal 

 end of the choroid fissure. The second objection is found in the phylogenic stages of 

 degeneration indicated in different eyes, notably that of Typhlomolge. Further- 

 more, it would not account for the groove that is undoubtedly found along the ven- 

 tral side of the larval eye, nor would it account for the presence of the inner reticular 

 layer around the optic nerve. It would moreover make it necessary to assume that 

 the cells found about the entrance of the optic nerve in Chologaster (Fig. 50) have 



