VERTBBEATES FROM A ORUSTAGEAN-LIKE ANCESTOR. 438 



I have come, then, to the conclusion that the nerve is in 

 connection with the right ganglion habenulse, and originally 

 passed straight to the surface from the posterior part of that 

 ganglion. By the shifting forward of the eye and brain wall 

 the nerve also was bent forward until it became almost hori- 

 zontal in position. That such a shifting of position actually 

 took place is nearly certain from the observations of Scott 

 (11), who describes how the eye which was originally behind 

 the ganglion habenulse has shifted forwards during the deve- 

 lopment of the young animal. Ahlborn also mentions the 

 same shifting of position. 



Further, in the eyes of the Arthropoda the optic nerve 

 passes into the optic ganglion, which is a well-defined struc- 

 ture separate from the rest of the supra-cesophageal ganglia. 

 The optic ganglion is composed of a cortical layer of small 

 nerve-cells closely packed together, and an internal medullary 

 portion composed of nerve-fibres which form, according to 

 Patten (26), the medullary stalk of the optic ganglion by 

 which it is connected with the brain, and form also the medulla 

 of the ganglion itself, which bears special relations to the eye 

 belonging to that optic ganglion. 



In ray opinion the right ganglion habenulse is the optic 

 ganglion of the dorsal pineal eye. It is composed of an internal 

 medullary portion of nerve-fibres, and a cortical portion of 

 small nerve-cells closely packed together of the same kind as 

 the berry-like cells of the grey matter of the brain, resembling 

 therefore the cells of the optic ganglion as given by Patten 

 (25). If I am right in my belief that the nerve of the pineal 

 eye can be followed into the right ganglion habenulse, it appears 

 to me to lose itself in the central medullary mass of fibres. In 

 the innermost portion of the ganglion the medullary part is 

 the main portion From it are formed the large Meynert's 

 bundle which is traceable to the ventral side of the brain in 

 the neighbourhood of the large ventral fissure, also fibres which 

 connect together the two ganglia habenulse forming the com- 

 missura tenuissima of Ahlborn (which perhaps becomes 

 the commissura mollis of human anatomy). A third set 



