95 



"der Recessus neuroporicus unmittelbar hinter den Columnae fornicis 

 liegen müßte, so daß die Fornices dann ebenfalls noch zur Lamina 

 infraneuroporica gehörten" appears a much more rational explanation 

 of the state of affairs. 



That portion of the prosencephalon to which the olfactory ganglion 

 is apphed to form the bulbus olfactorii is another easily recognisable 

 landmark in the cerebrum of any Vertebrate. Lying between the mesial 

 portion of the olfactory bulb and the situation of the anterior com- 

 missure is a grey mass, equally easy to recognise in the Amphibian, 

 the Sauropsidan or in certain lowly Mammals. If the mesial wall of 

 the olfactory bulb be traced backwards in a cerebrum (5 mm long) 

 of a foetal Platypus, it will be found to be directly continuous with 

 the mesial hemisphere wall, which shows a clear histological differen- 

 tiation into two horizontal parts. The larger inferior part is the 

 "precommissural area", a large prominent grey mass, very unlike the 

 corresponding region in the adult, but very like that of certain Rep- 

 tiles. Posteriorly it is directly continuous with the lamina infraneuro- 

 porica, which it closely resembles histologically, morphologically and 

 probably functionally in the adult. Dorsally it is directly continuous 

 with the second horizontal band, which forms a thinner narrower part 

 of the mesial hemisphere wall, which passes backwards above the 

 foramen of Monro and fissura choroidea. This band is the anterior 

 extremity of the hippocampus, the part pordering on the precommis- 

 sural area being the anläge of the fascia dentata. Behind the lamina 

 terminalis, the fascia dentata forms the dorsal border of the fissura 

 choroidea. This anterior extremity of the hippocampus, which is 

 arranged around a shallow Bogenfurche, does not bend down to the 

 base of the brain, but becomes directly continuous with the 

 olfactory bulb almost immediately behind the place where the 

 olfactory ganglion ceases. From this it is evident that the fissura 

 prima of His (which divides the lobus olfactorii into two lobules) 

 cannot be strictly considered part of the Bogenfurche. The latter 

 (the Ammonsfurche of Mihalkovics) has a definite morphological 

 significance — corresponding exactly in the non-placental Mammal to 

 that area, which is to become the hippocampus. As the pallium in- 

 creases in extent in the development of the Mammalian brain, it 

 appears to tend to push the hippocampus downwards. Where the 

 latter forms the margin of the hemisphere i. e. in the greater part of 

 its extent, it becomes folded in the complex manner which characterises 

 this part of the Mammalian brain, to overcome this tendency. The 

 rapid increase in the fascia dentata, which is the marginal part, has 



