No. 2.] VERTEBRATE CEFHALOGENESIS. 24 1 



ently of each other, though I have no hesitation in venturing 

 the prediction that this will be found to be the case. 



M. The parietal-pineal eye of the Cyclostomata and other 

 vertebrates has been developed from a median portion of the pig- 

 mented eye of Amphioxiis. The rudiments of this eye were de- 

 rived from {segmental) sense organs, but the eye itself is never 

 developed from two right a?id left halves, in so far as the closure 

 of the m.edullary folds would necessitate this. 



No absolute demonstration of this view is at present possi- 

 ble, but I wish to offer the following considerations in sup- 

 port of it. 



1. The parietal eye of vertebrates is formed as a hollow out- 

 growth of the anterior portion of the roof of the primitive 

 thalamocoele, and its cells contain pigment. 



2. The pigment contained within the cells of the bulbar, or 

 functional portion of the pineal eye, is derived phylogenetically 

 from the median portion of the pigment body of the anterior 

 end of the primitive thalamocoele of an Amphioxus-like form. 



3. The genetic connection of these three organs, for light- 

 perception and sight, viz., the two lateral eyes and the median 

 unpaired eye, is furthermore rendered probable by the central 

 connection of all three organs, which is found to be in the optic 

 thalami. 



4. The same essential elements are present in eacn, — pigment 

 cells and percipient rods and cones. 



5. All three organs were formed to supply the demand for 

 the restoration of the more perfect conditions for light-percep- 

 tion, destroyed by the folding in of the medullary canal. 



6. The two kinds of eyes were primarily alike in structure 

 and lensless; both formed lenses, the paired eyes first, — and this 

 condition has been retained by all descendants, the pineal eye 

 only in certain forms, — ancestors of the groups still showing 

 traces of the lens body, and their descendants. The double 

 nature of this organ, recently described in Leydig and Selenka, 

 does not affect the above views, for, so far as we yet know (and 

 the matter needs further investigation), both of the tubular out- 

 growths, although intimately related, are materially different in 

 one respect, viz., only one, the epiphysis, contains pigment in its 

 distal portion. 



