64 JOURNAt Of COMPABlATIVfi NeUROLOQV. 



ter." Again p. 179, "It will be in harmony with these obser- 

 vations to account for two distinct structures, when they occur, 

 by reference to this known case of the existence of two pairs 

 of accessory optic vesicles, in front of the mid-brain reg;ion, on 

 the cephalic plate of Squalus acanthias. Whether or not simi- 

 lar Anlagen for epiphysis and pineal eye exist clearly defined in 

 other animals, can be determined only after renewed observa- 

 tion upon them." 



In the final paper we read : "I have not been able to de- 

 termine whether it is the epithelium of the first pair of acces- 

 sory vesicles only, or whether epithelium from the second pair 

 is also included. Either the epithelium of the two pairs is in- 

 corporated into the walls of the thalamencephalon by being 

 carried together, or the epithelium of the second pair fades into 

 the surrounding substance of the brain wall, which almost im- 

 mediately grows into the vesicle of the mid-brain." 



It would thus seem that the fate of the second pair of ac- 

 cessory vesicles is likewise unsettled. 



When all these questions are satisfactorily settled in accord 

 with the author's views, there remain serious obstacles to the 

 homologies offered. Ritter has called attention to some of these 

 in the following words : 



"Let it be fully established that the ancestral verte- 

 brate possessed two or more pairs of serially homologous eyes 

 situated on the dorsal side of the central nerve axis, and we 

 should have a good starting point for an explanation, not only 

 of the epiphysis and pineal eye, but also of certain structures 

 found associated with them. (The accessory structures, which 

 it may be anticipated will find their explanation in this direc- 

 tion, are i) some of the collateral evaginations from the roof of 

 the developing brains of several groups of vertebrates ; 2) the 

 parapineal organ present in Petromyzon, and in some lizards, as 

 the young of Anguis fragilis (Prenant, '93) ; 3) as a reversion 

 the parapineal organ in an occasional adult individual of Phryn- 

 osoma coronata.) It must be recognizedj however, that as our 

 knowledge now stands there are some rather serious difficulties 

 to be overcome before the explanation will be complete. With 



