242 



tinues caudad along the border of the telencephalon medium. This 

 gray matter appears in the figures as a clear space between the two 

 parts of the tractus pallii and surrounding the crossed tract. In the 

 very narrow part of the telencephalon medium (Figs, 16—20) the 

 gray matter disappears and the strand is continued as a bundle of 

 fine non-medullated fibers. In Figs. 21 and 22 these fine fibers are 

 shown going up with the tractus taeniae to the commissura superior. 



Fig. 27. Reconstruction of the forebrain tracts dealt with in this paper, projected 

 upon the median sagittal plane. 



Here they cross to go to the primordium hippocampi of the other 

 side, thus constituting a true commissure. This corresponds to the 

 posterior pallial commissure of selachians, amphibians and reptiles and 

 probably a similar commissure is present in cyclostomes, ganoids and 

 teleosts. Wherever it has been clearly recognized it is a commissure 

 of the hippocampus or the primordium hippocampi. 



Kappers has very correctly pointed out that "Von den Holo- 

 cephali läßt sich einerseits der Typus des Ganoiden- und Teleostier- 

 Vorderhirns, andererseits derjenige des Selachier- Vorderhirnes leicht 

 ableiten." However, on account of his error in identifying the tractus 

 taeniae, comparison of the forebrain of Chimaera as Kappers de- 

 scribes it with that of other fishes is quite impossible. I shall hope 

 to show at another time how the forebrain of Chimaera does afford 

 great help in the attempt to discover the genetic relations between 

 the different types of forebrain in fishes. 



