THE PIXEAL BODY 59 



It is part of the small mass of cells situated between the base 

 of the pineal gland and the first fold of the chorioid plexus. The 

 unpaired eye is an evagination of the dorsal wall of the inter- 

 brain and constitutes an optic vesicle. The separation which 

 sometimes occurs between the crystalline and retina of this 

 vesicle is ordinarily unilateral, rarely bilateral. It appears 

 relatively late in embryonic development and should not be con- 

 sidered a proof of the duality of origin of the parietal organ as 

 Beard 18 has considered it. The unpaired eye does not occur in 

 chordates nor does it have its homologue in the other branches 

 of the metazoa. Sometimes it has its physiological analogue in 

 the median eye of Crustaceans. It is an ancestral organ which 

 was atrophied in the majority of extant forms of the different 



Fig. 24 Two successive stages in the development of the epiphyseal complex 

 in Lacerta vivipara, according to Francotte, 1896. 



Pa., parapineal organ; Po., pineal organ; M., midbrain. 



branches of the chordate phylum. The primitive optic vesicle is 

 still recognizable in cyclostomes and Saurians ; it is rudimentary 

 in teleosts and amphibians, but appears to be absent in sela- 

 chians. On the other hand, the epiphysis in these latter forms 

 is very long and broadened at its distal extremity without form- 

 ing an optic vesicle. The epiphysis is also derived from an 

 evagination of the interbrain roof. It does not represent the 

 optic pedicle of the parietal eye. It is an organ sui generis 

 whose function is still unknown. It reveals no marked sensory 

 characteristics even in selachians where it is markedly devel- 

 oped. It appears in the entire series of vertebrates and is an 

 ancestral organ. The paired eye and epiphysis appertain to the 

 interbrain while the paraphysis is part of the endbrain. This 



