587 



e Pi 



cp.e 



Fig. 4. Transverse section of the two cerebral 



hemispheres of the same in the plane a, b 



(fig. 3). cp. i, capsula interna. 



Mt/rmecobius , and Notoryctes^ even though many of these genera 

 possess larger brains than some of the üiprotodonts. 



These facts seemed to suggest that the aberrant bundle 

 was essentially a distinctive feature ofthe Diprotodont 

 Marsupials, and it ap- 

 peared to me that the cru- 

 cial test of this hypothesis 

 would be afforded by the 

 examination of the brain of 

 Thylacinus^ which although 

 that of a Polyprotodont, is 

 almost, if not quite, as large 

 as the brain of the largest 

 Macropod, and considerably 

 larger than those of all other 

 living Diprotodonts. I ac- 

 cordingly submitted the ce- 

 rebrum of TJiylacinus to the 

 test, and found no trace of 

 the aberrant bundle (figs. 3 

 and 4), wherefore it is 



clear that the presence of this aberrant fasciculus ofthe 

 ventral commissure is distinctive of the Diprotodontia. 



If we compare the brain of the Diprotodontia with that of the 

 other three Mammalian groups: Monotremata, Polyprotodontia, and 

 Eutheria, the meaning of the aberrant bundle becomes, I believe, 

 fairly obvious. 



A study of the structure of the brain in the Monotremes and the 

 Polyprotodont Marsupials shows that in the progenitor of the Mam- 

 malia all the commissural fibres of the neopallium must have passed 

 into the ventral commissure via the external capsule (fig. 4). 



The most pronounced growth tendency in the earliest Mammals 

 must have been the enormous increase of the extent ofthe neopallium, 

 for while at the beginning of the Eocene period this was almost as 

 insignificant as it is in the Keptilia, in most recent Mammals it attains 

 a bulk which far exceeds that of the whole of the rest of the nervous 

 system. This sudden expanse of the neopallium would lead to the 

 development of an enormous mass of fibres which must find some 

 outlet from the pallium. There are only three possible routes for 

 commissural fibres ofthe neopallium to the mesial plane. There is first 

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

 such fil)res passing to the ventral commissure: we find the second 



40* 



