471 



out principally by means of the Weigert, Pal, Golgi and anilin blue 

 black staining methods. The fibres of the external olfactory root 

 arise as axis-cylinder processes of the "mitral cells" of the olfactory 

 and terminate by means of "collaterals" and end-branchings in re- 

 lation with the protoplasmic processes of the cells of the pyriform lobe 

 (uncus) and olfactory tract as demonstrated already by Ramon t 

 Cajal ("N eue Darstellung vom histologischen Bau des 

 Centrain er vensystem s", Archiv f. Anatomie und Physiologie, 

 Heft VI, 1893). No fibres can be traced by this route into the hippo- 

 campus, as stated by Edinger and Herrick (loc. cit). 



The external root and the middle root (which goes to the pes 

 pedunculi) will not be considered in this paper. Before describing the 

 internal root, some reference must be made to the relations of the 

 mesial hemisphere wall in Ornithorhynchus. In front of the 

 anterior commissure there is an area of grey matter {p.a Fig. 1) 





Kf 



•^' 



dLL-vJ&jGk- . 



o[{1U 





Fig. 1. Mesial aspect, cerebrum of Platypas; enlarged. 



which is directly continuous behind with the thickened lamina terminalis 

 which lies between the anterior commissure and the hippocarapal 

 commissure {h. f) and which is the homologue of the septum pellu- 

 cidum of higher mammals. Above, this region is directly continuous 

 with the fascia dentata below with the "quadrilateral area" of 

 Broca and in front (below) with the mesial olfact. root {in. o. r.). 

 This area is developed in Platypus from the posterior olfactory 

 lobule of this and is homologous to the gyrus subcallosus of 

 Zuckerkandl — the peduncle of the corpus callosum of human 

 anatomy. Both of these terms are inapplicable to the monotreme 



