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but also of certain structures found associated with them. (The 

 accessory structures, which it may be anticipated will find their 

 explanation in this direction, are 1) some of the collateral evagin- 

 ations 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 

 Phrynosoma coronata.) It must be recognized, however, that as our 

 knowledge now stands there are some rather serious difficulties to be 

 overcome before the explanation will be complete. With reference to 

 the accessory evagination, the paraphysis of Selenka, the anterior 

 epiphysis of Etcleshymer, the ependyma of Hoffmann, all are in 

 front of the epiphysis proper, while in Squalus, according to Lacy, 

 it is the pair of eye cups behind the ones that develop into the 

 epiphysis that is wholly lost by degeneration. Of course it does not 

 necessarily follow that because the first pair behind the lateral eyes 

 becomes the epiphysis in Selacheans the same would be true in 

 Teleosts, where we know (Hill, '91) that an anterior secondary 

 evagination forms and has a transient existence; nor in Amphibians 

 where the same thing takes place (Eycleshymer '92). 



In Lacertilia the difficulty is made still greater by the fact that 

 the secondary evaginations are not only in front of the epiphysis, but 

 also that in some species (Leydig, '91) there is more than one. The 

 difficulty of position likewise holds in the case of the parapineal organ 

 in Phrynosoma, for here also it is situated in front of the pineal eye. 

 And here it may be mentioned that the relation of the two organs 

 to each other in this species is an obstacle in the way of a precise 

 comparison of the parapineal organ here with that described by 

 Prenant in Anguis, where he regards the posterior, smaller vesicle 

 as the accessory one. These difficulties with reference to the position 

 of the accessory organs, whether vesicles or evaginations, taken together 

 with the fact brought out by Lacy that the epiphysis has primarily 

 a bilateral origin suggests the question of whether the parapineal 

 organ may not be the lateral mate of the pineal eye. This 

 suggestion receives considerable support from the conditions presented 

 by Petromyzon and Iguana. In the former Studnicka ('93 a) points 

 out that Ahlborn's ('83) supposition that the secondary vesicle arises 

 as an outgrowth from the larger dorsal one, is not conclusive. He 

 himself does not believe it to have such an origin, but thinks it arises 

 independently from the brain wall, though he has not traced its 

 earliest stages of development. He confirms the observation of Ahl- 



