472 



and marsupial brain since they possess no corpus callosum. I shall 

 therefore distinguish the region as "the precommissural area". 

 This area has been called "Septum pellucidum" or "Fornix- 

 Leiste" in reptiles by Edinger, the "intraventricular lobe" 

 by Herrtck, Meyer was unwilling to call it "septum pellucidum" in 

 reptiles, and Hill (Cerebrum of Ornithorhynchus — Phil. Trans. 

 1893) very doubtfully homologised it with the septum pellucidum in 

 Platypus. From the investigations of Marchand and Paul Martin, we 

 know that" the Septum" developes from the thickened lamina terminalis, 

 so that it cannot be homologous with the precommissural area which lies 

 in front of it. The quadrilateral area" of Bruca, the "precommissural 

 area" and the lamina terminalis (septum pellucidum) are directly 

 continuous with one another and present similar histological features, 

 being composed of irregularly scattered medium sized pyramidal cells 

 (Fig. 2 d) whose highly branched axis cylinder processes extend 

 towards the surface, where they come into relationship with the 

 "collaterals" and terminal arborizations of fibres of the internal ol- 

 factory root (Fig. 2 b, Fig. 1 in. r. r.). The axis cylinder process 

 of some of these cells extends upwards in the mesial hemisphere wall 

 and in the anterior extremity of the hippocampus may be traced 

 into the alveus to end in the hippocampus (Fig. 2 e) by terminal 

 arborizations in relation to the protoplasmic processes of the pyra- 

 midal cells (h). Others enter the fimbria, probably to terminate in 

 a similar manner further back in the hippocampus. Others again, 

 especially those arising in the "septum pellucidum" cross in the 

 hippocampal commissure to the hippocampus of the opposite side. 

 The further connections of these regions shall not be considered in 

 this paper. The extensive grey mass consisting of the precommissural 

 area, septum pellucidum and "quadrilateral space" serves as] an 

 "intermediate station" between olfactory lobe and the hippocampus on 

 both sides. The fibres described probably correspond to the "Riech- 

 bündel" of Zuckerkandl and extend backwards to their destination 

 in the septum lucidum in placental mammals. 



Above these fibres in the precommissural area , lies a compact 

 bundle of fibres very well seen in sagittal sections of the cerebrum 

 of Platypus. In such a section through the mesial wall of the 

 Platypus brain, stained by Weigert's method, meduUated fibres may be 

 seen extending upwards and backwards in their whole length from the 

 olfactory lobe to the hippocampus. The uppermost of these enter 

 the fascia dentata directly, the lowermost — a much denser bundle 

 enters the fimbria, to terminate further back probably in the same 



