292 SCIENCE PROGRESS 



ordinarily sporadic manner, being confined to the Petromy- 

 zontidse amongst cyclostomes, and the Rhynchocephala and 

 Lacertilia amongst reptiles. In the myxinoids, the true fishes, 

 the Amphibia, the Crocodilia, the Ophidia, the Chelonia, the 

 birds and the mammals we can no longer recognise them 

 as special organs of sense, even if they be present at all. 1 

 We shall find, further, that either one or the other of the 

 original pineal outgrowths assumes dominance over its fellow 

 and becomes much more highly developed, while at the same 

 time it acquires a median position ; or possibly in some cases 

 the two have united together to form a median structure, 

 of whose bilateral origin no trace remains, at any rate in the 

 adult. 



Amongst the Petromyzontidae the pineal organs have been 

 investigated in the European lampreys of the genus Petromyzon 

 by Ahlborn (1883), Beard (1888), Owsjannikow (1889), Gaskell 

 (1890), Studnicka (1893 and 1899), Retzius (1895) and Leydig 

 (1896), while to Kupffer (1894) we owe the most detailed account 

 of their development. Johnston (1902) has given us some 

 observations as to the corresponding organs in the American 

 Lampetra, and the present writer has lately (1907) published 

 a full account, of them in the New Zealand Geotria (fig. 2), 

 upon which the following description is mainly based. In all 

 these cases there is a striking agreement in structure and 

 relationships of the parts under discussion. There are two 

 pineal organs, an anterior (L.P.E.) and a posterior (R.P.E.). 

 The latter is always much more highly differentiated than the 

 former, and actually lies in the middle line. In Petromyzon it 

 lies on top of the anterior organ, but it is a significant fact that 

 in Geotria the anterior organ lies in front of and a little to the 

 left of the posterior. The posterior pineal organ is developed 

 as a hollow outgrowth of the brain-roof, immediately in front 

 of the posterior commissure (C.P.). It grows forwards above 

 the roof of the brain, and its distal extremity enlarges to form 

 the " pineal eye." The originally hollow stalk of the outgrowth 

 solidifies, and nerve-fibres (P.N.) appear in it which connect the 



1 If we may believe the evidence afforded by the presence or absence of a 

 parietal foramen, the sporadic character of the distribution of the '' pineal eye " 

 seems to have been no less pronounced in past geological epochs, for while 

 in existing Rhynchocephala {Sphenodoii) it is very well developed, in the closely 

 related Hyperodapedon and Rhynchosaurus of palaeozoic or early mesozoic age no 

 trace of a parietal foramen exists, as I am informed by Dr. Smith Woodward. 



