PINEAL EYE IN LACERTILIA. 227 



resemblance in position or structure to the pineal eye of 

 Lacertilia. 



As figured by Langerhans ^ and Nusslin,^ it is a pigment 

 spot within the walls of the neural canal, and lies anterior to 

 the part shown subsequently by Hatschek to be the anterior 

 neuropore ; whereas if it were the homologue of the azygos eye 

 of Tunicata it must lie posteriorly to this. 



Turning to the Urochorda a structure is at once met with 

 which naturally suggests comparison with the pineal eye. 

 Yet, however tempting it may be to homologise the azygos 

 Tunicate eye with the latter, it cannot be too clearly pointed 

 out that the two organs differ fundamentally in 

 structure and position, and we have not the slightest 

 reason for supposing that the pineal eye is the direct repre- 

 sentative of the Tunicate eye. In the first place, the internal 

 position of the latter clearly distinguishes it from the pineal 

 eye; even supposing the tunicate organ to, in some way, undergo 

 evagination there still remains the difficulty that the retina cor- 

 responds in position to the part which after evagination would 

 give rise to the lens, whilst the latter structure is perfectly 

 distinct in nature and formation from that of the pineal eye. 



The curious formation of the lens in Tunicates from the 

 union of two or more separate parts, difi'ering in shape and quite 

 distinct from that of Lacertilia in their relationship to the 

 retina, is an important point of difference, and renders it quite 

 impossible, whatever may be the case with the retina, to homo- 

 logise the lens in the two forms. At the same time there is 

 considerable analogy between the two lenses, inasmuch as each 

 is formed directlyout of the walls of the neural canal, 

 a point in which they at once agree with one another, and differ 

 from every other Vertebrate. Notwithstanding this it must, I 

 think, be admitted that the vesicular nature of the eye in 

 Lacertilia and the formation of the lens out of a portion of 

 the vesicle, constitutes a difference of fundamental importance 

 between the two eyes in their fully- developed condition. 



' ' Arch. f. Mikr. Anat.,' Bd. xii, Tf. 12, fi^. 17. 



^ ' Zur Kritik ties AuipLioxusaugea,' Utto Nusslin, Tubingen, 1877. 



