230 Cerebral Commissures in certain Marsupialia. [Mar. 5. 



commissural fibres of the neopallium to the mesial plane. There is 

 first of all the external capsule, which chiefly consists in all Mammals 

 of such fibres passing to the ventral commissure : we find the second 

 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 : — 



Commtssura 

 dorsalis. 



Commissura 

 ventralis. 



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

 figs. 2 and 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 different mammalian brains. 



All the neopallial commissural fibres in the Polyprotodontia and 

 some only of these in the Diprotodontia and Eutheria follow the 

 first, which is also the primitive, route (a). The commissural fibres, 

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

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

 and pursue the second route (b). In the Eutheria the neopallial 

 commissural fibres from the dorso-lateral region of the hemisphere 

 forsake both the first and second routes and break through the hip- 

 pocampal 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- 



