24 Journal of Comparative Neurology. 



description of the relation of the olfactory radices in all the 

 species at our disposal. In a brain of a very young Aiiiiurus, 

 which by accident had been so obliquely cut that the plane of 

 section coincided with the curvature of the radix lateralis, the 

 entire course could be seen in a single section and it was, further- 

 more, evident that no fibres were or could be given off between 

 the pero of the olfactory tuber and the hippocampal nidulus, as 

 there was a definite boundary on the dorso-lateral aspect. The 

 radix lateralis is therefbre a distinct and direct tract of the pero. 

 Moreover, careful examination with oil immersion lenses showed 

 that the fibres of the radix mesalis terminate in cells of the pes; 

 i. e. in the small fusiform cells which form the axis of the tuber. 

 The evidence seems therefore to be complete not only that these 

 fibres have a different central destination, but that they arise in 

 distinct parts of the tuber and have nothing in common. If the 

 pes olfactorii be regarded as a prolongation of the basal part of 

 the prosencephalon, the radix mesalis (entalis) is simply a special 

 part of the praecommissura. 



A comparison of European fishes shows that here too the 

 regions in the cerebrum described above are obvious internally 

 if not externally. To describe all the slight variations encoun- 

 tered has at present perhaps no sufficient purpose, but it is in- 

 teresting to see that the cephalic part of the axial lobe always 

 receives fibres exclusively from the ventral peduncle so that the 

 suggestion that it contains the homologues of the anterior cor- 

 tical regions receives an important substantiation. 



It has proven possible to trace the fibres from both dorsal 

 and ventral peduncles into the cells. 



In all cases except the superficial zone of horizontal or 

 oblique cells and the large multipolar cells of the central lobe 

 the apex of the cell directed/'w// the 7'enfruk pa.'^ses directly into 

 a strong fibre. The course of the fibres can also be followed 

 with all desired distinctness. This fact has a very important 

 bearing on the interpretation of cerebral structure in other 

 groups. As I shall endeavor to show elsewhere, we have a sim- 

 ilar arrangement in the amphibia. It is in fact the primitive 

 arrangement in all cases. 



This is perhaps the place to mention a small nidulus radicis. 

 lateralis consisting of a few cells of very large size situated on 



