32 Journal of Comparative Neurology. 



the end-nucleus, and perhaps represents the corpus interpedun- 

 culare of authors. All the rest of the nucleus consists of cells 

 closely adjoining the ventricle as in Acipenser. Caudal to the 

 decussation the bundles of fine fibers at the right and left con- 

 tinue close to the ventral surface, ai: first diverging and then 

 approaching one another. In both Golgi and haematoxylin 

 sections there appears a second decussation some distance be- 

 hind the first, formed by the bundles of fine fibers (Fig. 2). 

 Caudal to this second decussation, which reaches nearly to the 

 end of the nucleus, there is still a very distinct small compact 

 bundle of fine fibers on the right side. This continues cau- 

 dalJy, crosses as a bundle to the left of the raphe, and runs on 

 nearly to the caudal end of the medulla. The course of this 

 bundle, together with the existence of the second decussation 

 formed by the fine fibers, indicate that the fibers are being dis- 

 tributed to their endings and it is therefore difficult to believe 

 that they are the fine fibers which end freely in the ganglion 

 habenulae. They are traced much farther caudally here than 

 has before been done. There is a certain similarity between 

 this bundle and the biDidlc x described in Acipenser and the 

 question arises whether that bundle may not after all be the 

 continuation of the bundles of Meynekt. I regret that a com- 

 parison of the Acipenser and Petromyzon pieparations throws 

 no further light on this question. 



Ahlbokn traced these bundles only to the point which I 

 have spoken of as the second decussation, and did not recog- 

 nize an end-nucleus. I have suggested ('01 c, p. 99) that the end 

 nucleus may be regarded as a special group of slightly modified 

 commissural cells. The greater elongation of the nucleus in Pe- 

 tromyzon and the fact that the bundles extend through nearly 

 the whole length of the medulla lend support to this view. It 

 seems probable that as the bundles pass backward in the me- 

 dulla the fibers gradually find endings in relation with the com- 

 missural cells. 



Ahlborn describes tracts entering the ganglia habenulae 

 from the thalamus and from the lateral expansions of the fore 

 brain ("hemispheres"). F. Mayer ('97) describes only the 



