468 THE PINEAL ORGAN 



regression of the organ had already commenced, and that it had ceased 

 to function as a visual organ. 



4. In some palaeozoic fishes, e.g. Pholidosteus, Rhinosteus, and 

 Titanichthys, bilateral pineal impressions are visible, either (a) on the 

 dorsal or outer aspect of the pineal plate, or (b) on its inner or intracranial 

 surface. Moreover, evidence of the bilateral nature of the pineal system 

 is also present in existing species. Thus in some species in which two 

 separate parietal eyes are present, e.g. Petromyzon or Geotria, each eye 

 is connected by its own nerve with the habenular ganglion of the same side ; 

 and when the two parietal organs differ in size there is a corresponding 

 difference in size of the habenular ganglion and also of the fasciculus 

 retroflexus of Meynert of the two sides. 



5. In those animals in which there is normally only one parietal 

 sense-organ or an unpaired epiphysis, the normal connections of the 

 basal part of the stalk of the parietal organ or of the epiphysis with the 

 right and left habenular ganglia and posterior commissure are bilateral. 

 Moreover, the occasional occurrence of accessory parietal sense-organs 

 and indications of coalescence of two retinal placodes, or of two lenses 

 in a single eye, may also be regarded as evidence pointing to a primary 

 bilateral origin of the system. Bifurcation of a single pineal stalk into 

 two terminal vesicles has also been observed as a variation in different 

 classes of vertebrates, more particularly in fishes (Cattie) ; in amphibia 

 (Cameron) ; in reptiles (Spencer, Klinckowstroem) ; in birds, e.g. 

 Emys europea (Nowikoff ) ; and among mammals several instances in 

 human embryos. 



6. The development of two separate pineal diverticula, in the median 

 plane and in the interval between the habenular commissure and the 

 posterior commissure, seems to be a rare occurrence, although two 

 terminal vesicles which have arisen from a common stalk may lie one 

 behind the other. If one parietal vesicle only is developed and it is later 

 cut off from its stalk of origin, the latter is usually displaced backwards 

 so that the epiphysis lies behind the parietal eye. Apart from the para- 

 physis, which originates anterior to the velum transversum, diverticula 

 arising from the roof of the third ventricle in front of the habenular 

 commissure are developed from the dorsal sac or postvelar arch, and give 

 rise to the suprapineal recess or are an outgrowth from the choroid plexus. 

 Neither the paraphysis nor diverticula originating from the postvelar 

 arch are epiphyseal in nature. 



7. The parietal eye, which seems to have attained its maximum 

 development in certain extinct amphibia, reptiles, and mammal-like 

 reptiles, and the epiphysis or pineal body usually show signs of regression 

 in specimens of mature living species. The most important of these 



