539 



Ramon y Cajal (op. cit., p. 22) protests against Meyer's and 

 ray statements that a tendency to the differentiation of a fascia dentata 

 was manifested in some reptiles and the results of his own and his 

 brother's histological investigations certainly demonstrate that there is 

 no properly -constituted Fascia dentata in the Reptilia. All that I 

 maintain (although Levi goes further) is that there is in the margin of 

 the hippocampus of Reptiles some indication of the commencement of 

 changes, similar to those which lead to the formation of the fascia 

 dentata in the Mammalia. The photograph of a section through the 

 brain of Lepidosiren (Fig. 2) affords very clear evidence that the 

 margin of the pallium, even in this lowly vertebrate, differs from the 

 rest of the column of cells. 



Summary. 



1. If the features of the brain in the Dipnoi be considered as a 

 whole, they will be found to approxioiate much more nearly to those 

 of the Amphibia than to those of any other Vertebrata. 



2. In the cerebral hemisphere of the Dipnoi there is a definite 

 pallial formation or cerebral cortex, distinctly separated from the 

 ependyma as a clearly-detined layer of nerve cells 



3. This pallial formation is homologous to the pallium of the 

 Amniota. 



4. Although its distinctive layer of cells is much more definitely 

 segregated than the homologue of the pallium is in the Amphibia, 

 and is compactly arranged as a distinct layer of cells, so that it 

 resembles the pallium found in the Reptilia, it is neither so densely 

 packed, nor so highly specialised nor so well differentiated as the 

 reptilian cortex. 



5. The most outstanding feature of the Dipnoan brain is the 

 relatively enormous development and high degree of specialisation of 

 the tuberculum olfactoriura. 



6. This fact, taken in conjunction with the important role of the 

 sense of smell, as observed by Graham Kerr in the living Lepidosiren, 

 points to the conclusion that the tuberculum olfactorium is the dominant 

 organ of the Dipnoan nervous system. 



7. The contention of Burckhardt and Bing that the cortex of 

 the tuberculum olfactorium in the Dipnoi is the representative of the 

 cerebral cortex of the Amniota and that it is the "prosencephalon 

 sensu strictiori" receives no support from my investigations. 



