360 H. M. BERNARD. 
have to face the embryological histories of these eyes and the 
interpretations usually put upon them. 
Taking the definitive eye first, it is found to arise ontogeneti- 
cally by the union of two distinct structures. The ‘ primary 
optic vesicle,” as it is called, develops as an invagination of 
the brain towards the skin. This collapses into itself so as 
to form a cup or spoon, open towards the skin,and joined to 
the brain by a stalk or handle. The anterior wall, lining the 
hollow of the cup or spoon, becomes the retina and retinal 
palisade layer; the posterior wall, forming the outside wall of 
the cup or spoon, becomes the pigmented epithelium in contact 
with the palisade layer. Into this cup there dips down from 
the outer skin an invagination which becomes constricted off 
and forms the lens. The outer rim of the cup or spoon grows 
over the lens so as to form, with other elements, the iris. 
The question we have to try to answer is, are these pro- 
cesses even approximately historical, or are they purely em- 
bryological adaptations in order to obtain a desired end by a 
series of short cuts? The answer must depend entirely upon 
the weight of the evidence. It must be remembered that we 
have no direct evidence whatever. Our sole guides are infer- 
ences to be drawn from known facts, and these we are only 
roused up to use when discussing some proposed clue to the 
right understanding of the said facts. In the present instance 
a theory of light sensations compels me to assume that the 
Vertebrate eyes have developed from the skin, and in the fore- 
going pages I have endeavoured to show that the known facts 
of structure and function are explicable on this assumption. 
The morphology and physiology of the eye, as the latter is 
interpreted by the theory, go throughout hand in hand. When 
we turn to the embryological development our connected 
story is thrown into confusion. Some of the details, it is true, 
are in most satisfactory agreement, as we shall presently see; 
but the most essential processes, viz. those just described, 
differ entirely from our scheme. 
In the face of this serious difficulty, are we to withdraw or 
boldly to examine the embryological processes in order to 
