530 



the whole formation must be practically given up to the function of 

 smell. It is only in more highly-organised vertebrates, when other 

 sensory impulses, such as those of touch and sight, are poured into 

 this pallium and the olfactory function is relegated to its fringe, that 

 a central non-olfactory "Neopallium" can be said to come into existence. 



A comparison with the conditions found in the Amphibian, Rep- 

 tilian and Mammalian brains can leave us in no doubt as to the iden- 

 tity of the corpus striatum in Lepidosiren, even though it is no thicker 

 than the pallium. It is that part of the lateral wall which is on the 

 ventral side of the edge of the pallium (Fig. 1, A). At the same time 

 it is not possible to draw a line of demarcation between it and the 

 tuberculum olfactorium. 



The most curious feature of the Dipnoan brain is the large sau- 

 sage-like, ventral appendage of the hemisphere, which Burckhardt 

 calls "Lobus postolfactorius". In Fig. 1 I have labelled it "Tuber- 

 culum olfactorium". When I began the study of the brain of Lepido- 

 siren the interpretation of this feature of the hemisphere seemed to 

 present insuperable difficulties, for it is quite unlike anything that I 

 had seen in any other vertebrate. Burckhardt's various descriptions 

 of it as "postolfactory lobe", „Hirnrinde" (op. cit. Taf. 3, Fig. 22), and 

 "Ammonshorn" (Taf. 3, Fig. 21) did not help in the solution of the 

 problem. 



But, once the olfactory bulb, the paraterminal body and the pyri- 

 form lobe had been located, an area was thus circumscribed, which 

 could not be anything else than the tuberculum olfactorium. Even 

 though this part of the brain in Lepidosiren is relatively so enormous 

 and so highly developed there is no escape from the conclusion that 

 it must be the homologue of the tuberculum olfactorium. Another 

 very surprising feature of this area is the close resemblance its 

 structure presents to the homologous region of the Mammalian brain. 



based on the descriptions of P. Ramon, Van Gehuchten and Bochbnek. 

 In my first account of this tract I called it "the olfactory bundle of 

 the fascia dentata" (Anat. Anz., Bd. 10, 1895, p. 473) and two years later 

 (Journ. of Anat. and Physiol., Vol. 32, p. 39) substituted the shorter 

 name "Fasciculus marginalis". At that time I thought it was a direct 

 connection between the olfactory bulb and the hippocampus; but since 

 then I have discovered that it is interrupted by a synapsis in the 

 olfactory peduncle. Thus its arrangement agrees with that of the 

 Tractus olfacto-corticalis of the Amphibia. I have discussed this iden- 

 tity here to emphasize the fact that not only in structure, topographical 

 position and relationships, but also in its fibre connections, the hippo- 

 campus of the Ichthyopsida resembles that of the Mammalia. 



