DEVELOPMENT OF THE EYE OF AMBLYOPSIS. 161 



The Second Period. — The development during the second period is direct and 

 leads to the condition obtaining at the end of that period. Some of the processes 

 are palingenetic, some are of purely ontogenetic significance, while still others (if 

 I may make the distinction) are degenerative. 



The optic nerve develops at the beginning of the period in an undoubted phylo- 

 genetic way. As in the case of the eye as a whole, the nerve develops directly into 

 its full size. The details of its history are given under the head of the optic nerve. 

 The latter half of the history of the lens belongs entirely to this period. Its his- 

 tory is also given under another head. The changes the lens undergoes during 

 this period are all katagenic, and some time before this period closes the lens has 

 disappeared. 



The direct development of the optic vesicle of the beginning of this period into 

 the eye as found at the end of this period is very difficult to interpret satisfactorily. 



A comparatively very narrow marginal part of the secondary optic vesicle is 

 converted into the epithelial part of the iris. The lens is almost always entirely 

 excluded from the optic cup when the iris develops. The extreme shallowness of 

 the optic cup and the comparative thickness of the retina would lead one to expect 

 the choroid fissure proper to be a very short structure. The shallow cup develops 

 into the adult eye by processes like those that take place in normal eyes. These 

 purely palingenetic processes operating on so deficient material give rise to condi- 

 tions that are not palingenetic. 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 complicated by the fact that the vitreous cavity has ontogenetically 

 disappeared nearly as much as phylogenetically, so that, while the processes of 

 changing the optic cup into the eye are palingenetic, the material operated upon 

 being quite different from that normally obtaining in fish embryos, the resulting 

 stages of the eye are not palingenetic. 



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 center of the Amblyopsis eye is thus the result of the phylogenetic 

 rather than the ontogenetic 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 homologucs of the cells found 

 at the point of entrance into the eye of Chologaster (fig. 65, z). This would imply 

 that a cavity has not disappeared from the center 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 Mo. This 

 interpretation is especially suggested by figure 62, c. This would account for the 



