20 LIVERPOOL BIOLOGICAL SOCIETY. 
The wall of the vesicle is cellular throughout, and its 
anterior or external portion is thickened to form a cellular 
lens (see Pl. I., fig. 8, U.), while its posterior or internal 
part is modified to form a retina (7.) composed of several 
layers of cells, of which those next to the cavity of the 
vesicle are rod-like and pigmented, while those lying 
outside are continuous with the fibres of the nerve going 
to the brain. Spencer concludes that this structure is an 
organ of sight, which, though probably not functional in 
any of the living forms in which it has been investi- 
gated, was an important median sense-organ in the extinct 
Vertebrata of Pre-Tertiary times. 
Among the theoretical conclusions at which Spencer 
arrives from his investigations on the pineal eye there are 
two which seem to call for remark. The first is, that, 
although he points out De Graaf’s mistake, in comparing 
the epiphysis with a molluscan eye such as that of the 
Cephalopoda, still in several passages he himself describes 
it as an invertebrate eye, or an optic organ of the 
invertebrate type. This I think is scarcely allowable, 
considering :— 
1. That the epiphysis and its eye are an outgrowth 
from the nervous system, the cavity of the vesicle being 
the remains of a part of the cavity of the neural canal, 
2. That the lens in the pineal eye is cellular, being 
formed from the descendants of some of the epiblast cells 
of the wall of the neural canal, 
8. That the retina is likewise derived from epiblast 
cells of the wall of the neural canal. 
In fact the only point in which the pineal eye resembles 
the invertebrate type is in having the epithelial layer of 
the retina directed towards the source of light and the 
nervous layer towards the nerve supply. | 
The three diagrams (figs. 2 to 4) on Pl. I. represent :-— 
