. Anatomischer Anzeiger. Anatomy, Comparative. 387 the alveus arises posteriorly, because it is cut obliquely. That this apparent neopallial origin of the most caudal alveus-fibres can have nothing to do with the formation of the corpus callosum is shown by the fact that a similar appearance is presented in horizontal and sa- gittal sections of the infrasplenial part of the hippocampal formation in those mammals possessing a definite corpus callosum, which can be shown to be distinct from this supposed neopallial bundle of the alveus. It is also exhibited in those ventral parts of the hippo- c
. Anatomischer Anzeiger. Anatomy, Comparative. 387 the alveus arises posteriorly, because it is cut obliquely. That this apparent neopallial origin of the most caudal alveus-fibres can have nothing to do with the formation of the corpus callosum is shown by the fact that a similar appearance is presented in horizontal and sa- gittal sections of the infrasplenial part of the hippocampal formation in those mammals possessing a definite corpus callosum, which can be shown to be distinct from this supposed neopallial bundle of the alveus. It is also exhibited in those ventral parts of the hippo- campal formation in the Marsupialia (see Zuckerkandl's Fig. 3, op. cit. N. lc), which cannot possibly be concerned in the formation of the corpus callosum. If a corpus callosum really exists in Perameles, we should expect to find the most certain and unequivocal evidence of its presence in that region where it is known (vide supra) to de- velop first (in the Eutheria) and in sections which are cut in the same plane as its fibres must pursue. The figure illustrating this account has been chosen because it fulfils all these requirements. It is, moreover, ty- pical of the whole hippocampal region in the Marsupialia in that area which is affected in the Eutheria by the process of callosal development. A glance at this sec- tion shows that the vast majority of the fibres of the alveus certainly come from that part of the hippocampal formation which exhibits (in a sec- tion stained by Weigert's method) the characteristic clear band (see Figure) representing the densely packed layer of pyramidal cells. A small group of fibres also enters the alveus from the region (x) which intervenes between this typical hippocampus and the typical neopal- lium. In this transition- region (x) the pyramidal cells are not densely- packed in a close column as they are in the typi- ?A&dl^ •*>«, .cU*k. &„}«* r Ut Fig. 1. Coronal section of the right dorsal hippocampus of Perameles o
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