PIGMENTATION OF THE EYES OF VERTEBRATES 71 



The appearance of a central spot inside the white rim is due to the inter- 

 position of the lens between the cornea and the retina, obscuring a direct 

 view of the central part of the retina. In reviewing these points we may 

 note that : when the pineal eye is removed and the parietal cornea is then 

 viewed from above, the central white area has disappeared ; and if the 

 parietal cornea is held up to the light, the area is seen to be translucent. 

 When sections of the pineal eye are examined with the microscope by trans- 

 mitted light, the white pigment contained in the retinal cells of older 

 specimens is found to be absent in young specimens or specimens which have 

 been fixed in acid-containing fluids . In adult specimens or late larva; fixed in 

 alcohol or other preservative not containing acid, the " granules (? phosphate 



L.P.E. 





mm$ r 



Fig. 47. — The Head of 

 an Adult Lamprey, 

 Petromyzon p/aneri, 



SEEN FROM ABOVE ; 

 SHOWING THE PARI- 

 ETAL Area and Pari- 

 etal Spot (p.s.) 

 (After Studnicka, 

 1893-) 



R.P.E. 



Fig. 48. — The Pineal and 

 " Parapineal Organs " 

 (Right and Left Pari- 

 etal Eyes), as seen from 

 above under a dis- 

 SECTING Lens. (After 

 Dendy, 1907.) R.P.E. , 

 L.P.E. : right and left 

 parietal eyes. Geotria. 



of lime) appear as minute spherical bodies evenly distributed through the 

 inner and greater part of the outer segment of the cells " (Dendy). 



The inheritance of a negative character, such as the generalized 

 absence of pigment in albinism or the absence of a particular constituent 

 of the blood or of a ferment such as prothrombin in cases of haemophilia, 

 is well known ; but the inheritance in living animals of a localized absence 

 of pigment in connection with the vestige of what presumably was once, 

 in animals belonging to the palaeozoic period, a functional organ, is a 

 problem of the greatest interest, and the solution of this problem appears 

 to be as difficult and as remote as many other problems relating to pigment 

 distribution and the inheritance of pattern. 



A significant exception to the absence of pigment in the region of 



