72 THE PINEAL ORGAN 



the " parietal scale " occurs in the wall-lizard {Lacerta muralis), in which 

 pigment has been shown by Leydig not only to lie beneath the parietal 

 or corneal scale but even to encroach on the clear central spot which lies 

 immediately over the pineal organ. This change is evidently a secondary 

 one, and is probably due to the need for pigment in this situation in a type 

 of animal frequently exposed to the heat of the sun and lacking any 

 protective covering in the way of hair or feathers. The present needs 

 of the animal in this case have apparently overcome the inherited bias 

 towards lack of pigment formation in the pineal region, but it is possible 

 or, indeed, probable that the pigment figured by Leydig is not actually 

 formed in or under the corneal scale of the lizard, but has been carried into 

 the region by chromatophores from the surrounding tissues ; if this is 

 the case, the hereditary lack of pigment formation in the area still exists, 

 but it has been countered by the migration of pigment cells into the deeper 

 layers of the cornea. 



Pigmentation of the Pineal Region in Fishes 



The parietal sense-organ, or " pineal eye," is generally thought to be 

 absent in living classes of fishes ; though it is possibly represented in 

 some by the expanded end- vesicle of the pineal organ. Notwithstanding 

 the absence of the parietal sense-organ, there is frequently a parietal 

 foramen, or a parietal canal or pit, closed externally but opening into the 

 cranial cavity internally (Fig. 49). This lodges the " end- vesicle " and 

 distal part of the stalk of the pineal organ. Over this area is a pigment- 

 free area of the epidermis and corium. In the spiny dog-fish {Spinax 

 niger), the " Scheitelfleck " is very conspicuous, although no true parietal 

 cornea is present, like that of the cyclostomes and certain reptiles. It 

 appears as a pale area in the parietal region, which contrasts sharply with 

 the deeply pigmented skin surrounding it. This area is separated by a 

 considerable thickness of loose, subcutaneous tissue containing the tubular 

 organs, or ampullae of Lorrenz, and it is completely separated from the 

 pineal organ by the cartilage closing the parietal canal. 



In some fishes, e.g. Raja clavata, the stalk of the pineal organ is 

 accompanied throughout its whole length by a sheath containing blood- 

 vessels ; this shows an intensive degree of pigmentation, especially around 

 the large dorsal vein, which is wider than the diameter of the end-vesicle 

 itself. 



In the spoonbill, Polyodon (Fig. 50), a lozenge-shaped parietal plate 

 is present over the end-vesicle of the pineal organ, and the skin over this, 

 as in Spinax, is destitute of pigment. The general relations of the pineal 

 organ to the cranial wall are very similar to those of Spinax. 



Very few allusions are made with reference to the existence of pigment 



