356 H. M. BERNARD. 
humour was almost entirely lost (Diagram IV), compensation 
would be found in the great increase of the inner surface of 
the eye, the posterior half of which alone could function as 
retina; the other, anterior half, consists of the undifferentiated 
palisade epithelium, which allows the pigment to pass 
through as slime into the vitreous humour. It is also pro- 
bable that some material for the same purpose finds its way 
along the optic nerve into the cavity of the eye. 
This suggested origin of the vitreous humour, viz. that it is 
largely due to pigment granules passing into the eye cavity 
and dissolving into slime, is not only in accord with our 
homology of the retina with the epidermal palisade layer which 
gives rise to the mucus layer, but it is also supported by 
certain facts. In sections of the eyes of embryo chickens, 
dendriform exudations of slime, obviously forming the vitreous 
humour, are seen coming from the ciliary portion of the retina. 
The vitreous body is known to be easily detachable from all parts 
of the interior of the eye, except the ciliary region and the 
place of entrance of the opticnerve. The pigment granules are 
known to pass from the pigmented epithelium into the palisade 
layer in the iris, and the process can be seen beginning before 
the lens is reached. The actual microscopic appearances in 
fortunate sections are all in favour of their supplying slime 
for the vitreous humour. In my series of sections of the larva of 
Petromyzon planeri the retina is only in contact with the 
pigmented epithelium in the axis of the eye, only here are any 
rods and cones found ; where the retina is not in contact with 
the pigmented epithelium, not only are no rods and cones 
developed, but the pigment does not accumulate in the choroid 
epithelium. It appears as if, there being no layer of rods and 
cones to hinder its. advance, the pigment granules pass 
freely through, and form masses of semi-clarified granules 
within the space between the retina and epithelium. In the 
ciliary region of the retina, where the two layers—the retina 
(without rods) and the epithelium—come gradually into con- 
tact, all the appearances are as if the pigment granules are 
streaming through the retina into the vitreous humour. The 
