ope H. M. BERNARD. 
have lengthened greatly, and bent over the specialised area 
somewhat in the manner indicated. These long cells would 
therefore be merely a modification of the ordinary prickle- 
cells. By the detachment and death of their distal portions 
they would be able to yield a succession of horny cells, so as 
to maintain a continuous hard covering over the retina. This 
covering appears eventually to have formed a primitive surface 
lens, moved by fibrils from the intrusive connective tissue. 
The next stage in the development of this hypothetical 
epidermal eye was probably an invagination of the whole of 
the modified portion of the skin, including the lengthened 
palisade or lens cells. This invagination is shown in Diagram 
III. Two factors may have helped to bring it about. 1. The 
retina becoming bulged inwards by the great accumulation of 
slimy fluid in the cavity of the eye, the tendency of the 
surrounding skin would be to constrict off the vesicle so 
formed. 2. The lens being attached by fibrils to the place of 
entrance of the optic nerve, and probably slightly moveable, 
would tend to be drawn in with the retina. Any way, if the 
eye started at all as we have suggested, we are justified in 
assuming an invagination such as that described, for that alone 
appears able to explain the arrangement of the tissues, among 
which I would call attention to the loops of the vascular system 
of the cutis, which, diagrammatically represented, assume the 
positions. shown by faint dotted lines in Diagram III.! 
Further, the remains of the neck of such an invagination may 
still perhaps be seen in the fibrous ring which persists after 
the formation of the aqueous chamber as the ligamentum 
annulare (fishes) or ligamentum pectinatum (other Vertebrates), 
and which, it is important to notice, connects the iris (and not 
the lens) with the cornea (lig. an., Diagram IV). 
If we picture to ourselves the change of shape which the 
spoon-shaped retina shown in Diagram II would have to 
undergo in passing to the form shown in Diagram III, the 
origin of the choroidal fissure is at once apparent. As the 
1 In this I am following the familiar diagrams of the blood-vessels of the 
Vertebrate eye to be found in text-books. 
