218 W. BALDWIN SPENCER. 



organ of the Invertebrate type, whilst from the walls 

 of a precisely similar vesicle, and within the same 

 animal, may be formed an eye of the Vertebrate 

 type. 



In both cases the nerve-fibres enter into connection with 

 the retinal elements lying on the side remote from the rods ; 

 in one case, however, important secondary developments take 

 place which are wanting in the other, and to which are further 

 entirely due the differences existing between the two types of 

 eyes. 



In the case of the pineal eye, first, we have a vesicle, the 

 anterior portion of the walls of which are transformed into the 

 lens; of the cells forming the walls of the posterior half, those 

 facing into the cavity give rise to the rods, whilst external to 

 these are formed the other retinal elements, into connection 

 with which enter the fibres of the pineal stalk ; the primary 

 optic vesicle persists, and there is thus formed an eye on what 

 is usually spoken of as " the Invertebrate type,^' i. e. the rods 

 facing directly into the cavity of the vesicle, and the nerve 

 entering into connection with the external lying elements. 



In the case of the paired eye, however, we find that, whilst 

 up to a certain point it agrees in development precisely with 

 the pineal eye, after that point is reached secondary struc- 

 tures appfear which have an important influence on its final 

 form. The retinal elements are foi;med out of the cells of the 

 vesicle wall ; the lens, however, is not, but arises as an invagi- 

 nation which pushes before it the external wall; whilst there is 

 this difference between the lenses of the two forms, we see at 

 once, when dealing with the retinal elements, that they are 

 formed in a similar position to that in which they are present 

 in the pineal eye — that is, the cells facing into the optic 

 vesicle give rise to the rod-elements, whilst the external 

 lying cells give rise to what are really the outer layers of the 

 retina (nuclear and molecular layers, &c.). It is simply the 

 formation of the lens as an invagination which causes the rods 

 to assume what appears to be an external position, but is 

 external only when regarded in connection with a secondarily 



