461 



connective-tisse fibres of the pia mater, I have not succeeded in fol- 

 lowing them continuously to the brain in the adult animal. 



In several series of sections of embryos of diHerent ages, however, 

 the nerve has been traced to the brain as one continuous bundle of 

 fibres without difficulty, and it is quite clear that it enters the left 

 habenular ganglion. It becomes closely attached to the roof of the 

 dorsal sac, however, before it reaches the habenular ganglion — or 

 the spot where this will be developed — and this fact probably ex- 

 plains why the lower part of the nerve is broken up into separate 

 strands in the adult, for the rapid growth of the thin wall of the 

 dorsal sac may be supposed to cause the spreading out of the nerve- 

 fibres over its surface. 



Nerve-fibres first appear in the retina of the pineal eye while 

 the latter is still resting upon the brain-roof, and I have come to the 

 conclusion that they grow from the retina to the brain as in the case 

 of the lateral eye. 



A curious feature of the nerve of the pineal eye in the adult 

 animal is that it receives bundles of nerve- fibres from the wall of 

 the pineal sac as well as from the eye itself. This point has already 

 been noted by Gisi. 



The left habenular ganglion in the adult is produced upwards to 

 meet the wall of the dorsal sac in a characteristic manner at a point 

 where it receives nerve-fibres from the latter in special abundance. 

 The right habenular ganglion also receives fibres from the wall of the 

 dorsal sac, but is not produced upwards to the same extent as the 

 left one. This asymmetrical development of the habenular ganglia 

 further supports the conclusion that the left habenular ganglion is 

 especially associated with the pineal eye. 



It is extremely difficult to form any conclusion as to how far 

 the pineal eye of Sphenodon still functions as a light-percipient organ. 

 Such experiments as have hitherto been made have yielded entirely 

 negative results. The concentration of a bright light upon the skin above 

 the pineal eye elicits, so far as I have been able to make out, no 

 response; but then it must be remembered that the animals are ex- 

 tremely sluggish, and a similar experiment with the lateral eye may 

 be continued for some time without producing any visible effect beyond 

 the contraction of the iris. 



Structurally, the only sign of degeneration which the pineal eye 

 exhibits is to be found in the very large amount of pigment present 

 in it in the adult, for I do not think we need regard the degeneration 



