350 Robert J. Terry. 



of the mid-brain nor in the diencephalon, but is so situated that 

 its posterior layer stands in connection with the former, while 

 its anterior stratum is spread out in the roof of the latter caudad 

 to the epiphysis. In form and position, therefore, it differs from 

 the corresponding tract of the elasmobranchs, in which fishes 

 it has been shown to be a fiber bundle associated wholly with the 

 midbrain (Ehlers, 78; Edinger, '99; Minot, '01). With this differ- 

 ence in the commissures of the two classes of fishes is correlated the 

 difference presented by the pars intercalaris, which is extensive 

 in the teleosts, small or absent in the elasmobranchs. 



Regarding a pineal nerve, the evidence was insufficient to war- 

 rant the statement that such a structure is present in Opsanus. 

 There is that close relationship between the posterior commissure 

 and the epiphysis which has been described by Edinger ('99) in 

 Scyllium and sturgeon, by Kupffer ('93, '06) in the trout, and 

 shown by Dean ('96) in his figure of Amia (Comp. fig. 11). In 

 the smaller embryos of Opsanus the posterior commissure extends 

 further upon the base of the epiphysis than it does in the adult, 

 recalling the relation of a tractus pinealis. Many fibers were 

 followed to the base of the epiphyseal stalk but their terminal 

 relations were not discovered. 



Superior Commissure 



The observation made by Osborn ('84) on the position of the 

 superior commissure in front of the epiphysis has been many 

 times confirmed and in recent years emphasized by Minot ('01) 

 and Dexter ('02). " Cameron ('04), however, appears unwilling 

 to concede that the commissure should be considered as closely 

 related to the epiphysis. He states that in all vertebrates it is 

 situated behind the root of the choroid plexus of the third ventricle. 

 The definition of the position of the commissure given by Osborn, 

 Minot and Dexter implies a topographical relation to a constant 

 organ. The structural relationship between the commissure and 

 the epiphysis in the ganoids described by Herrick ('91) and by 

 Eycleshymer and Davis ('97) is another reason, beyond that of 

 mere topography ,for associating the two parts. Yet, the position 



