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to this known case of the existence of two pairs of accessory optic 

 vesicles, in front of the raid-brain region , on the cephalic plate of 

 Squalus acanthias. Whether or not similar Anlagen for epiphysis and 

 pineal eye exist clearly defined in other animals, can be determined 

 only after renewed observations upon them, but, even if they be 

 lacking, it will be negative evidence and their undoubted presence in 

 one animal will still be significant. Morphologists no longer expect 

 to find a complete record of the steps of phylogeny in any one animal. 

 The little understood conditions of development have led to the sup- 

 pression of significant traces in one animal while they have been pre- 

 served in another. There are also slurrings of structures, obliterations 

 and modifications in all animals. The Elasmobranchs are a primitive 

 group and it cannot be reasonably doubted that we have preserved 

 here a clue to a primitive mode of origin of the pineal eye. 



It was mentioned, at the beginning of this article, that there is 

 some evidence to show the existence of three outgrowths from the 

 roof of the embryonic fore-brain, the epiphysis, the vesicle of the 

 pineal eye, and the paraphysis. If we can admit that the first two 

 may be accounted for in the way I have suggested, we then have 

 left the paraphysis. That structure arises from the prosencephalon 

 and is more likely connected with the appearance of the choroid 

 plexus than a structure homologous with the epiphysis. 



Summarizing the facts recorded in this paper we reach the 

 following conclusions: 



1) There are preserved on the cephalic plate of Elasmobranchs 

 (Squalus acanthias) at least two pairs of accessory optic vesicles. 



2) These, taken with the primary optic vesicles, give to the 

 embryo three pairs of rudimentary eyes. 



3) The anterior pair develop into the lateral eyes, and the first 

 accessory pair form the walls of the thalamencephalon , and give rise 

 to the principal outgrowth from it. 



4) The epiphysis is, therefore double in origin, forming from a 

 united pair of accessory optic vesicles. 



5) Since the latter are homologoues with the lateral eyes the epi- 

 physis, their derivative, is also homologous with the lateral eyes. (Its 

 differences in structure need to be explained.) 



6) It is highly probably that the enlarged distal end of the 

 epiphysis in Squalus is homologous with the pineal eye in those forms 

 in which it is differentiated. 



Lake Forest University, 



Nov. 15, 1893. 



