PARIETAL ORGAN. 



71 



persists and assumes a peculiar structure resembling that of the 

 retina of the eye. For this reason it has been called the pineal 

 eye. In Lampreys the pineal body also assumes the same 

 structure. A great deal of significance has been attached to 

 the curious eye-like structure which is assumed by the parietal 

 organ. It has been regarded as the vestige of an unpaired eye. 

 In our opinion the resemblance to an eye is accidental, but for a 

 discussion of the question we refer the reader to the account of 

 the parietal organ in the section devoted to Reptilia. 



Fia. 39. Heads of young Elasmobranch embryos (Scyllium canicula) (after Sedgwick). 



A. Ventral view of head of embryo, 7 mm. in length, with two open pharyngeal clefts. 

 The mouth is present as a longitudinal groove in the ectoderm of the buccal depression. 



B. Same view of a slightly older embryo ; the buccal groove has become a longitudinal 

 slit. C. Side view of head of embryo, 9 mm. in length, with three open slits. D. Side 

 view of head of embryo, 11 mm. in length ; rudiments of external gills have appeared on 

 the hyoid and on the first and second branchial arches. E. Side view of head of embryo 

 of 16 mm. ; external gills have appeared on mandibular arch and the angle of the jaw is 

 marked. 1 mandibular arch ; 2 angle of jaw ; 3 second pharyngeal cleft ; 4 nasal pit ; 

 5 eye ; 6, midbrain ; 7, auditory sac ; 8 hyoid arch ; 9 spiracle. 



The superior commissure is a small nervous development in 

 the otherwise epithelial roof of the third ventricle just in front 

 of the attachment of the pineal stalk. The posterior commissure, 



the so-called parietal organ (pineal eye). See Dendy, Q. J. M. S., 42, 

 1899, p. 111. This view is supported to a certain extent by the arrange- 

 ment in the lamprey (see p. 106). 



