THE EYE-LIKE SPOTS IN FISHES. 
227 
to one another of the more highly developed animal forms ; hut 
which, we may say in passing, owes its existence neither to the 
laws laid down by Cuvier after a comprehensive survey of the 
adult, nor by Yon Baer after a close study of the characters of 
developing animals. 
To this distinction let us devote a little consideration. 
When we compare a section of the eye of any invertebrated 
animal with that of one of the Vertebrata, we find, with other 
subordinate differences, of which nothing need be here said, 
this most important and striking one ; in the Yertebrata the 
‘ rods and cones/ which are the terminal organs of the optic 
nerve, are placed quite at the back of the eye, are turned away 
from the light, whereas in the invertebrate, the Crayfish, for 
example, we find the anterior part of the end-organ just below 
the cornea, and as near to the light as possible ; not turned 
away from it, but towards it. 
This important difference in the optic arrangements of the 
adult of the two groups finds, like many other difficult problems, 
its explanation in the history of the development of the two 
kinds of eyes. Both have their prime origin in that outermost 
layer of the embryo — the epiblast— which is the veritable 
mother of all our sensations. In the invertebrate the only 
change which occurs is that the cells of this layer become 
modified and elongated ; in the vertebrate their history is more 
complicated ; firstly, the layer of epiblast dips in below the 
level of the back of the body, to form the spinal cord and 
brain ; this set of dipped-in cells get cut off from the exterior, 
and form a tube of epiblast within the body ; from the anterior 
region of this tube there proceeds an outward growth — the 
optic vesicle — which grows out towards that thickening of the 
epiblast which forms the cornea. Thus what was originally 
outside comes to be inside, and when it grows again to the 
surface, it does so with its constituent parts turned, as it were, 
inside out.* 
A reference to the subjoined diagrams (p. 228) will perhaps 
make this difficult point a little clearer.. If we take n and r as 
representing respectively the layers of nerve-fibres and modified 
retinal cells, we have in fig. A a representation of what obtains 
in the In vertebrata and in B of what is found in the adult 
Yertebrate ; C and C' will show the tube of epiblast, while in 
D we see the relation of the two layers in the developing optic 
vesicle, and E shows their position after the ‘ optic cup ’ is 
formed. 
The bearing of these considerations is not unimportant ; it 
leads us to ask what is really meant when the spots are spoken 
* We owe the first distinct literary statement of this history to Mr. F, 
M. Balfour, LL.D., F.R.S. (see his Elasmobrttnch Fishes, p. ICO). 
