cxxvi Journal of Comparative Neurology. 



gans develop coetaneously at the cost of two evaginations of the 

 intermediate brain. These are arranged one above the other and de- 

 tach themselves from the same cerebral region. This mode of formation 

 seems to me to speak in favor of a distinct genesis of two cephalic diver- 

 ticules represented, the anterior the parietal eye, the posterior the 

 epiphysis. The presence of a nerve independent of the epiphysis is very 

 strong proof for the individuality of the parietal organ. The authors 

 which have described more parietal organs have probably considered as 

 such \.\\e parap/iyst's of Salenka. 



"There are in the embryos of Saurians three encephalic diverticles ; 

 of these three a single one, the parietal organ, has played a sensory role. 

 These diverticles are not differentiations of the encephalon at the same 

 phylogenetic period. The visual functions of the parietal eye are atro- 

 phied at the same time that the dermic ossifications take on in the cranial 

 region a greater importance, and that the encephalon removes further 

 from the ectoderm. This eye is found among fishes, reptiles and also 

 among amphibians. It is rudimentary among the Teleosts. 



The ontogeny shows that the third eye appears after the closing of 

 the cephalic region of the medullary tube. It proceeds entirely from the 

 encephalon and swells into an optic vesicle, the dorsal wall of which 

 constitutes the crystalline humor of it and the ventral, the retina. 



But what has been the role of the parietal eye and why was it devel- 

 oped ? ' 



Eycleshymer answers this by saying that in the closing of the medul- 

 lary tube the equal or regular eyes — before this influenced directly by the 

 light — lost more or less completely their function. This loss of functional 

 activity was compensated for by the appearance of an azygos dorsal eye 

 which commenced to atrophy as soon as the two equal eyes began to re- 

 gain their preponderance. 



The conclusions which proceed from my review are as follows : 



1. The parietal cannot be considerad as a simple diverticle of the 

 pineal gland. In the Lacerta and Anguis it is a separate organ devel- 

 oped with and yet independent of the epiphysis. 



2. This eye is supplied by a bundle of transitory nerves not con- 

 nected with the epiphysis. It arises from a little cellular mass situated 

 between the base of the pineal gland and the fold of the plexus chor- 

 aideus. 



