222 W. BALDWIN SPENCER. 



able. At first sight Ahlborn's figures of the organ, especially 

 of the walls of the upper vesicle, call to mind the rod elements 

 of other forms, but a closer examination again reveals important 

 points of difference; they do not, as in Lacertilia, face into 

 the cavity, being bounded internally as well as externally by 

 nervous matter, and, more important still, there is an entire 

 absence of pigment, which is the prominent feature possessed 

 in common by the rods of all Lacertilian eyes. Further, again, 

 the cavity of the optic vesicle is traversed by strands of nervous 

 matter passing from the anterior to the posterior wall, a feature 

 entirely wanting in any pineal eye, however degenerate, amongst 

 Lacertilia. 



On the other hand, these rod-like structures occupy the 

 hinder wall of the vesicle, the proper position, supposing them 

 to be true but degenerate retinal elements ; and it may be 

 remembered that amongst Lacertilia, which must be regarded 

 as descended from ancestors possessed of pineal eyes, we do 

 know of one form (Cyclodus) in which the eye now stops at 

 a stage of development in which the cells of the posterior wall 

 much resemble those of Petromyzon, and are devoid of 

 pigment. The absence of lens also is paralleled in the case of 

 Cyclodus. 



(3) The organ is completely enclosed within the cartilaginous 

 cranium, and acquires a secondary connection with the brain 

 (its lower vesicle fusing with the left ganglion habenulae) 

 which is quite unknown amongst any Lacertilian. 



The conclusion to be drawn from these facts^ is that at the 

 present time the epiphysis of Petromyzon certainly does not 

 become modified into a pineal eye at all comparable to that of 



' For our knowledge of the structure of the epiphysis of Petromyzon we 

 must rely ou Ahlborn's description here quoted ; it is, of course, possible that, 

 viewed in the light of recent work, the structures described by him might be 

 found to bear another interpretation. I have not at present been able to study 

 the structure, but would suggest the possibility of what Ahiborn figures as 

 nervous material lying internal to the rod-like structures, and as strands of 

 tissue crossing from the anterior to the posterior wall of the vesicle, being in 

 reality only the coagulated remains of the fluid contents of the vesicle. 



