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 



r.H.S. Ut 



Fig. 1. Coronal section of the right dorsal 

 hippocampus of Perameles obesnla. X ^ • Weigert 

 stain. 



25* 



