588 



route in the path mapped out by the internal capsule from the dorso- 

 lateral neopallial area to it ; and the third route can only involve the 

 invasion of the alveus of the hippocampus. 



These three routes, by which a fibre coming from the dorsal neo- 

 pallium in the region x (fig. 5) may attain the region y in the other 

 hemisphere, are indicated schematically in the following diagram : — 



All the neopallial commissural fibres in the Polyprotodontia and 

 some only of these in the Diprotodontia and Euthcria follow the first, 



CommiKsarw 

 dorsaUs. 



Capsula 

 interna. 



Cnp.^ala 

 escUrna 



Commi ssura 

 ventrali s . 



Fig. 5. A scheme of a transverse section in the same plane as that represented in 

 fig. 2 and fig. 4 to show the three routes a, b, and c by which a commissural fibre may 

 pass from the point x in one hemisphere to the region y in the other in dift'erent 



mammalian brains. 



which is also the primitive, route (ai. The commissural fibres, which 

 spring from the dorso-lateral region of the neopallium in the Dipro- 

 todontia seem to be crowded out, as it were, of the first route and 

 pursue the second route [h). In the Eutheria the neopallial commis- 

 sural fibres from the dorso-lateral region of the hemisphere forsake 

 both the first and second routes and break through the hippocampal 

 formation (c), or, in other words, invade the alveus so as to form a new 

 dorsally situated neopallial commissure which is the corpus callosum. 

 This hypothesis of the origin of the corpus callosum I have pre- 

 viously stated in my memoir of 1894 (vide supra) and I discussed it 

 more fully in 1887 5. 



5 The Origin of the Corpus Callosum. Trans. Linn. Soc. of London, 2nd 

 series, Zoology, Vol. 7. part 3. June. 1897. p. Ü1. 



