86 THE ORIGIN OF VERTEBRATES 
thickening of the cuticular layer to form a lens could only exist as 
long as that layer is absolutely external, so that the light strikes 
immediately upon it; for, if from any cause the eye became situated 
internally, the place of such a lens must be filled by the structures 
situated between it and the surface, and the thickened cuticle would 
no longer be formed. 
In all vertebrates these pineal eyes are separated from the 
external surface by a greater or less thickness of tissues; in the 
case of Ammoccetes, as is seen in Fig. 31, the eye lies within the 
membranous cranial wall, and is attached closely to it. The position, 
then, of the cuticular, or corneal lens, as it is often called, on the 
supposition that this is a median eye of the arachnid type, is taken 
by the membranous cranium, and, as I have described in my 
paper in the Quarterly Journal, on carefully lifting the eye in the 
fresh condition from the cranial wall, it can be seen under a 
dissecting microscope that the cranial wall often forms at this 
spot a lens-like bulging, which fits the shallow concavity of the 
surface of the eye, and requires some little force to separate it from 
the eye. 
As will appear in a subsequent chapter, this cranial wall has 
been formed by the growth, laterally and dorsally, of a skeletal 
structure known by the name of the glastron. The last part of it to 
be completed would be that part in the mid-dorsal line, where appa- 
rently, in consequence of the insinking of the degenerating eyes, a 
dermal and subdermal layer already intervened between the source 
of light and the eyes themselves. 
When the membranous cranium was completed in the mid-dorsal 
region, it was situated here as elsewhere just internally to the sub- 
dermal layer, and therefore enclosed the pineal eyes. This, to my 
mind, is the reason why the pineal eyes, which, in all other respects, 
conform to the type of the median eyes of an arachnid-like animal, 
do not possess a cuticular lens. Other observers have attempted to 
make a lens out of the elongated cells of the anterior wall of the 
eye (my corneagen layer), but without success. 
Studni¢ka, who calls this layer the pellucida, does not look upon 
it as the lens, but considers, strangely enough, that the translucent 
appearances at the ends of each nerve end-cell represent a lens for 
that cell, so that every nerve end-cell has its own lens. Still more 
strange is it that, holding this view, he should yet consider these knobs 
