136 bulletin: museum of compara.tive zoology. 



fibre, but has described as such artifacts formed perhaps by pressure, or 

 rough haudhng, causing the myelin to flow out iuto the canal. In 

 Acanthias embryos of the age studied by Kalberlah, Reissner's fibre has 

 not yet formed in the canal (see p. 170). His studies, too, were appar- 

 ently confined to transverse sections of the cord, in which it would be 

 very difiicult to identify the fibre if it were present. We may therefore 

 safely set aside Kalberlah's conclusions, as it is improbable that he has 

 seen this structure. Eeissner's fibre has since been seen in selachians 

 by Houser (:0l), who confirms my findings, and by Johnston (:02) in 

 Petromyzon. Kolliker (:02, p. 159) further confirms the results of my 

 first paper as to the preformed nature of Reissner's fibre. Streeter 

 (:03) holds to the view that it is an artifact, as : " The structure sliows 

 a marked and irregular variation in form and size in different sections ; 

 in some transverse sections it was seen as multiple ' Centralfaden ' ; in 

 sections stained with toluidin blue it retains a deep blue 'stain, while the 

 axis-cylinders in all other parts of the section are unstained." The 

 appearances here described are evidently due to poor preservation of 

 the material. 



2. The Mesencephalic Nidulus' of Optic Reflex Cells. 



The nidulus of cells in the mesencephalic roof whose axons fuse to 

 form the fibre of Reissner were first described by Rohon ('77) in the 

 selachian brain, though they had previously been seen by Stieda ('70) 

 in the frog, and later in the turtle. Sanders ('86) described the position 

 and appearance of these cells in selachians with some minuteness. 

 Bellonci ('88) noticed them in the brains of the frog and the chick, and 

 Goronowitsch ('88) described cells in Acipenser which he recognized as 

 homologous with the ' Dachkeru ' of Rohon. Osborn ('88), studying 

 this nidulus in Amphibia, was the first to advance a theory as to its 

 relations. Falling into eiTor in following tlie neurites of the cells, he 

 • claimed that they were connected with the trigeminal nerve, aud called 

 this nidulus the ' mesencephalic trigeminal nucleus.' Barckhardt ('9l), 

 observing these cells in the Urodela, accepted the interpretation of 

 Osborn, as did Rabl-Riickhard ('94) for the Reptilia, thus perpetuating 

 the error. Sala ('95) gave an accurate but incomplete description of 

 the cells of the torus longitudinalis of teleosts, but failed to homologize 

 them with the corresponding cells in other vertebrates. Haller ('98) 

 found ill selachians and (:00) in Emys the ' Dachkern ' nidulus, and 

 claimed to have followed the neurites of the cells to the ventral longi- 

 tudinal bundles of the medulla. Mayer ('97) noticed large cells in 



