68 DK. a. ELLIOT SMITH ON THE 



" commissura clorsalis " a shorter path, but a clearer scope for longitudinal extension 

 than the ventral commissure provides. And tlms we have pallial fibres invading and 

 subsequently superseding the dorsal limb of what was previously the hippocampal 

 commissure. 



" Pallial " commissural fibres probably first make their appearance in Eeirtiles and 

 form a very insignificant constituent of the ventral commissure. In Monotremes and 

 Marsupials these fibres become extremely abundant, and swell the proportions of the 

 ventral commissure enormously. But in Eutheria a rapidly increasing proportion of 

 these fibres forsake the commissura ventralis and form the new " dorsal commissure of 

 the pallium " — the corpus callosum, — which throws the " parent " commissure into 

 insignificance. The remnant of the commissura ventralis is known in man by the 

 somewhat misleading name " anterior commissure." 



EXPLANATION OE PLATES 15 & 16. 



Fig. 1. Coronal section througli the forebrain of Peranicles nasuta. Stained by Weigert's method. 

 X4. The section passes just behind the commissures. 

 2. Coronal section through the hippocampal formation and the posterior extremity of the " com- 

 missura dorsalis^' in Peramehs. Wcigert stain. Fascia dentata represented as it appears 

 after nuclear staining, x 20. 

 Figs. 3 & 5. Coronal sections of the " commissura dorsalis " of Perameles, a short distance in front of 



fig. 2. X about fi. 

 Fig. 4. Representation of the commissures of Pernmek's (as seen in a mesial sagittal section) to show 

 the planes in which the sections represented in figs. 2 («), 3 {b), 5 (c), and 6 (d) w-erc cut. 



6. A coronal section of the cerebral hemispheres of Peram,eles, to show the general arrangement 



of the commissures. Wcigert stain, x about 6. 



7. Coronal section of the right cerebral hemisphere of Perameles immediately in front of the 



cerebral commissures. Weigert-Pal and carmine stains, x 4. 



8. Semi-schematic representation of the anterior part of the mesial wall of the cerebral hemisphere 



of Dasyurus vivervimis. X about 4. To show that, as a result of the unrolling of the hippo- 

 campus anteriorly, the whole of its morpholoyicully superficial layer now actually forms part 

 of the visible surface. Thus the surface of the hippocampus proper [hip.'") appears above the 

 fascia dentata [f.d.'). 



9. Anterior portion of the forebrain of Perameles, exposed in a mesial sagittal section. x 4^. 



The bulbus olfactorius only represented in part and in outline. 



10. Sagittal section through part of the anterior portion of the mesial wall of the cerebral 



hemisphere of Phascolarctos cinereus. Weigert stain. X G. 



11. Horizontal section through the left hemisphere of T/'icAosf^n/s ?;M/yje'c«/«, a short distance above 



the commissures, x 2. ant.mcs., the antero-mesial angle. 



12. Coronal section through the right hippocampal formation of a bat [Miniopterus Schreibersii) , to 



compare with tlie hippocampal region of fig. 1. Lithinm-carniine stain. x about 20. 



13. Coronal section o? Mi/iiopterus. Compare fig. 2. 



14. Coronal section of right hemisphere of Miniopterus. X about 20. Compare with fig. 6. 



15. The fascia dentata and adjacent parts. Enlarged from fig. 14. 



16. Coronal section of the liippocampal region of Miniopterus, a short distance in front of fig. 14. 



17. Sagittal section of the commissiires and adjacent parts of brain of Nyctophilus near the mesial 



plane. X about 40. 



18. Drawing of the " gyrus subcallosus " of a human bram. 



