364 H. M. BERNARD. 
more or less at right angles to the optic axis of the definitive 
eye, the two layers of the invagination becoming applied to 
form the more specialised elements of the organ. This is what 
we may assume to have been the first adaptive embryological 
process in the evolution of the Vertebrate eye. Its end was 
not to produce the definitive eye, but the hypothetical eye at 
the stage shown in Diagram II. Hence the primary optic 
vesicle may have originally been a simple ectodermal invagina- 
tion apart from the central nerve system. 
The earlier this retinal invagination is laid down, the more 
chance it would have of being involved in the medullary 
groove. Connection between them could be found, first of all, 
in the rudiments of the optic nerve, which would have been 
laid down as an ectodermal thickening joining the optic invagi- 
nation with the central medullary invagination. A further 
shortening of the process might soon lead to the primary optic 
invagination developing as a lateral offshoot from the medul- 
lary groove.} 
We are not altogether without evidence to support this in- 
terpretation of the origin of the primary optic vesicle, as an 
adaptation for the production of the eye when it was in a 
primitive condition,—such, for instance, as we have depicted in 
Diagram II. I have already called attention to what, accord- 
ing to the history of the eye here described, are certain very 
primitive features in the eyes of fishes. I refer to the accommo- 
dation of the eye by means of the m. retractor lentis (Beer), 
to the persisting power of moving the lens so as to vary the 
direction of sight, to the slight powers of movement of the 
eye itself as a whole, to the small development of the iris, 
and to the persistence of a prominent ring of tissue—the liga- 
mentum annulare—further forward on the iris than is the 
homologous ligamentum pectinatum of higher Vertebrates, 
which represent, according to the foregoing, the neck of the 
ocular invagination (Diagram III). Now it is surely of some 
1 The arrangement sometimes found, in which the optic vesicles only grow 
out after the groove has closed over, can be easily regarded as a secondary 
specialisation. 
