240 H. F. Osborn 



can be seen to enter the olfactory lobes. On the other hand it supplies 

 the dorso-medial portion of the mantle of the hemispheres from the 

 foramina of Monro to the boundary of the olfactory lobes. 



In all the Amphibia the olfactory lobes are defined externally 

 by the lateral swelling- of the surface. In horizontal brain sections, 

 there is also in most genera, an expansion of the ventricle in the 

 olfactory region. In Rana this expansion is surrounded by a com- 

 pact group of fine cells, outside of which are the olfactory glomeruli. 

 The upper and lower medial group of hemispheral cells may be 

 readily distinguished from the cells of the olfactory region. It is 

 thus that the scattering of the upper bundle is observed to lie en- 

 tirely within the hemispheres and not to extend to the olfactory lobes. 



In view of the above facts we can no longer regard the upper 

 bundle as the pars olfactoria of the anterior commissure, but rather 

 as a commissure of the dorsal portion of the mantle and 

 homologous on this ground with the corpus callosum. 

 The two divisions of the lower bundle may be compared with the 

 pars olfactoria and the pars temporalis respectively. This 

 view seems to me to be supported by their position in the brain 

 stem and their distribution, when we consider the pars olfactoria in 

 the relations which it has in the lower mammalia. Of the two, the 

 homology of the smaller division, the pars temporalis, seems less well 

 founded, until we study the reptilian brain. 



In answer to Stieda's objection that the upper bundle cannot 

 be compared to the corpus callosum because it lies in the floor of 

 the ventriculus communis, we may again refer to the Chelonian brain, 

 where this bundle has the same relative position as in the mammals. 

 In fact, the agreement between Stieda, Rabl-Eückhaed and 

 Bellonci in regard to the homology of the upper bundle with the 

 pars olfactoria has arisen from imperfect observation of its distri- 

 bution. With all deference to Bellonci's admirable work, I am at 

 present unable to verify his description of the upper bundle in the 

 Frog's brain (1882 Taf. II fig. 6). This bundle seems to me simply 

 a commissure and not a crossing of fibre tracts as well. All of its 

 fibres turn upwards and forwards into the hemispheres and not back- 

 wards into the optic thalami (see vertical sections). The bundle im- 

 mediately gives off fibres. The region in which it is distributed 

 anteriorly is not, as indicated by this author, a portion of the olfac- 

 tory lobes but rather a portion of the hemispheres. 



In regard to my comparison of the large lower bundle with tlic 



