66 Proceedings of the Royal Physical Society. 
that the pineal eye is developed, so far as it goes, altogether 
on the vertebrate type. We have, in fact, the main con- 
stituents of the vertebrate eye, viz., the epiblastic involution 
for future lens, and the primary optic vesicle from the brain 
arrested at a very early stage of development, viz., the stage 
where the external epiblast has responded to the call made 
on it from within by the outwardly directed process (pineal 
evagination) of the brain itself. 
It may be perhaps unsafe to conclude from the examina- 
tion of a single embryo, even though it be such a lowly 
organised vertebrate as a shark, that this is the true explana- 
tion of the involution of the epiblast, viz., for the purpose of 
forming a lens; but if this involution has not occurred in 
connection with a lens formation, it is difficult to explain its 
appearance in the embryo in any other way. 
On the other hand, it appears that the anterior portion of 
the pineal vesicle in Anguis fragilis is cut off to form a body 
resembling a lens. If this body, so separated, be really the 
morphological equivalent of a lens, then the pineal eye of 
Anguis, so far as the lens formation is concerned, is certainly 
not planned, if I may use such a term, after the vertebrate 
type. On the other hand, can the retina (so called) of the 
pineal eye be really looked upon as invertebrate in type ? 
There are reasons, in my opinion, against the view that the 
pineal eye is formed and fashioned after the invertebrate 
type :—(1) The retina is developed directly from the brain as 
an outgrowth from its upper surface after the formation of the 
primary cerebral vesicles. (2) If Dohrn’s views as to the 
phylogeny or origin of vertebrates be correct, then the pineal 
eye ought to be ventral in position, or at least we should 
expect the pineal stalk to arise from the under instead of 
the upper surface of the brain. If, then, the involution of 
the external epidermis over the pineal vesicle or body in ZL. 
cornubica be really the first step towards a future lens forma- 
tion—and, as I have shown, appearances point to this 
conclusion—it follows that the pineal body of all verte- 
brates is in reality a vertebrate type of eye, arrested, however, 
at a very early stage (primary optic vesicle stage) in its 
development. 
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