84 THE ORIGIN OF VERTEBRATES 
as already indicated, Patten looks on all these eyes as three-layered, 
and states that in all cases a corneagen or vitreogen layer exists, 
which gives origin to the lens, For my own part I agree with 
Patten, but we are not con- 
cerned here with the lateral 
eyes. It is sufficient to note 
that all observers are agreed 
that the median eyes are 
characterized by this well 
marked cell-layer, the so-called 
vitreous or corneagen layer of 
cells. 
This layer (¢., Fig. 35) is 
composed of much - elongated 
cells of the hypodermal layer, 
Fic. 36.—Eyr or Hypropryitus Larva, in each of which the large 
WITH THE PigMENT OVER THE RetinaL nucleus is always situated to- 
in ues : wards the base of the cell. 
i hisoes in comengen rage; Tho space hetween it and the 
end.cells, retina contains, according to 
Patten the cells of the pre- 
retinal layer (yr.). These may be so few and insignificant as to give 
the impression that the vitreous layer is immediately adjacent to the 
retina (ret.). 
Let us turn now to the right pineal eye of Ammoccetes (Fig. 37) 
and see what its further structure is. The anterior part of the eye 
is free from pigment, and is composed, as is seen in hematoxylin or 
carmine specimens, of an inner layer of nuclei which are frequently 
arranged in a wavy line. From this nucleated layer, strands of tissue, 
free from nuclei, pass to the anterior edge of the eye. 
In the horizontal longitudinal sections it is seen that these strands 
are confined to the middle of the eye. On each side of them the 
nuclear layer reaches the periphery, so that if we consider these 
strands to represent long cylindrical cells, as described by Beard, 
then the anterior wall may be described as consisting of long 
cylindrical cells, which are flanked on either side by shorter cells of 
a similar kind. The nuclei at the base of these cylindrical cells are 
not all alike. We see, in the first place, large nuclei resembling the 
large nuclei belonging to the nerve end-cells; these are the nuclei of 
