140 



for some iistaac^. The nucleus neuroporicua is a thiri olfac- 

 tory centre. 



The pallium has its t-^o halves fused in the median plane. 

 Its most important region is the pair of pallial eminences. 

 Sach of these has a special extension of a lateral ventricle. 

 The neurones of a pallial eminence ars not arranc^ed in layers, 

 but it is practicable to recognize three distinct forms: 

 (1) The neurones of the tractus pallii, the largest variety, 

 their axones comprising the tractus pallii. (2) Commissural 

 ani associative neurones, the axones of /?hich are distributed 

 in the pallium itself, a special decussation of commissural 

 fibres occurring in the median plane. (3) Cajal neurones, 

 lying tangential to the limitans externa. 



The pallium of Mustelas is regarded by the writer as a 

 primitive representative of the stem giving origin to the pallia 

 of amphibians, reptiles, birds, and mammals. It anticipates the 

 olfactory connections of the reptilian brain. The tractus pal- 

 lii is interpreted as giving the olfactory sense a hold on the 

 nervous system in addition to that provided by the epistriatum 

 and striatum, and the neuroporic and postolf actory nuclei. 

 The phylogenetic development of the pallium of selachians is 

 belie/ed to have been the outcome of the dreat dependence placed 

 upon the olfactory sense by these animals. 



