294 SCIENCE PROGRESS 



probably phosphate of lime, which constitutes the now well- 

 known " white pigment " so characteristic of the pineal eye 

 in the lampreys. There is no lens in the eye, and it seems 

 quite impossible that any image should be formed upon the 

 retina. The eye, however, lies immediately beneath a remark- 

 ably modified transparent area of the integument, so that it 

 can be actually seen through the skin in the living animal (at 

 any rate in P. Jluviatilis), and this fact, taken in connection with 

 its complex histological structure and its well-developed nervous 

 connection with the brain, indicates that it may still function as 

 a light-percipient organ. Experimental evidence is needed, 

 however, before this point can be satisfactorily determined. 



The anterior pineal organ (L.P.E.) is much smaller and 

 forms a kind of very imperfect copy of its fellow ; its lower 

 wall contains little or no pigment and no fully developed 

 sense-cells. It is connected with the left habenular ganglion, 

 upon the anterior extremity of which (G.H.L.A.), indeed, it lies, 

 and arises as a hollow outgrowth of the brain immediately in 

 front of the superior commissure (C.H.S.) by which the two 

 habenular ganglia are connected (Kupffer). 



In spite of the fact that the two pineal organs in the 

 lampreys actually arise one behind the other in development 

 and maintain these relative positions throughout life, there 

 are very strong grounds for believing that they are really 

 members of a pair, of which the right one (pineal eye, upper 

 epiphysial vesicle, posterior pineal outgrowth) is very much 

 better developed than the left (parapineal organ, lower epiphy- 

 sial vesicle, anterior pineal outgrowth). The evidence consists 

 chiefly in the very remarkable asymmetry of the parts of the 

 brain with which they are respectively connected. Thus the 

 right habenular ganglion is very much larger than the left one, 

 and so also is the right bundle of Meyne'rt (R.M.B.). Now the 

 ganglia in question are undoubtedly paired organs, 1 so also are 

 the bundles of Meynert connected with them, and their extra- 

 ordinary asymmetry can only be explained as the result of the 

 unequal development of the sense-organs associated with them. 

 It is, therefore, only reasonable to suppose that these sense- 

 organs must themselves have originally been bilaterally arranged 



1 Although, owing to the hypertrophy of the right ganglion and the forward 

 prolongation of the left one, they may appear one in front of the other in such a 

 section as is represented in fig. 2. 



