434 W. H. GASKELL. 



of fibres which arise from it are the conspicuous strands which 

 pass by way of the optic thalami to the cerebral lobes. These 

 latter form the medullary stalk of the optic ganglion, the optic 

 thalamus being that part of the supra-cesophageal ganglion 

 which connects together the brain proper and the optic gan- 

 glion. It is worthy of notice that all these strands of fibres 

 do not stain like good nerve-fibres^ but take on a peculiar tint 

 with various staining reagents as pointed out by Ahlborn. 

 Further, we see, from the series of horizontal sections that in 

 the more superficial sections in which the left ganglion habenulse 

 is only slightly or not at all involved, the internal medullary 

 part is divided into two portions, of which the one is composed 

 of the more superficial fibres of the commissura tenuissima 

 and tsenia thalami, while the other forms the medulla of the 

 ganglion habenulse itself. In fig. 27, PI. XXVIII, I give one 

 of a series of horizontal sections through the pineal eye and 

 ganglion habenulae of an Ammocoetes 66 mm. long, which was 

 stained whole in boro-picro-carmine. The peculiar shape of the 

 arrangement of the nerve-fibres which form the medulla of the 

 ganglion with respect to the nerve-cells is shown in the figure. 



In every respect, then, it appears to me the right ganglion 

 habenulse proves itself to be the optic ganglion of the dorsal 

 pineal eye. 



6. The Left or Ventral Eye. — There is, however, a left 

 ganglion habenulse and a left Meynert's bundle, very much 

 smaller than the right ; if this also is an optic ganglion the 

 eye belonging to it ought to be visible, though we should expect 

 to find it more degenerated, and its structure less easy to define 

 than the dorsal eye connected with such a vigorous optic ganglion 

 as is represented by the right ganglion habenulse. This second 

 eye is the epiphysis iii of Ahlborn^ i. e. the lower vesicle which he 

 describes as possessing a structure similar to that of the upper 

 vesicle, or epiphysis ii, which is now recognised as the pineal 

 eye.^ In this cavity, however, Ahlborn finds the remains of 



' He has evidently made a mistake in supposing that the lower vesicle 

 communicates with the upper ; all my sections show clcarlv that they are two 

 separate structures. 



