CEEEBEAL COMMISSUEES IX THE YERTEBEATA. 



471 



with grey substance derived from the paraterminal body, as in the Mouotreme ; and in 

 this grey mass a number of longitudinal nei've-fibres are found which constitute the 

 mesial elements of a larger series of fibres which are found iu that part of the para- 

 terminal body which clearly corresponds to the region distinguished as a in the foetal 

 Echidna. There can be no valid reason for refusing to regard these fibres as the 

 homologues of the '■'fasciculus maryinalis " of the Monotrcme. The fact that a foramen 

 or " intercommissural recess " is found in the " commissure-bed " in the Reptile and not 

 in the Monotreme, need cause no difficulty, because a sagittal section of the lamina 

 terminalis at once shows the unimportant nature of this feature : on the other hand, the 



Fig. lii. 

 Portion of a coronal section through the brain of a Monitor (Hijdrosaiinis). 



hip. 

 <a.L\'. 



yoara 



Scheme of lamina terminalis to show the plane of the section ,r-_i/ above. 



presence of such a recess between the columns of the fornix — the vecessus triangularis of 

 Scbwalbe — is a well-known fact in the case of the brains of the Meta- and Eutheria. 



The dorsal edge of the paraterminal body is fused with the lower extremity of a thin 

 portion of the wall of the hemisphere, which presents a peculiarly distinctive histological 

 structm'e. It is characterized by the presence of a very regular closely-packed column 

 of cells, which is placed somewhat nearer the ventricle than the surface of the hemi- 

 sphere. The component cells of this column are moderately large pyramids, the apices 

 of wdiich are directed toward the surface ; from the basal aspect of each cell an axis- 

 cylinder process passes off and contril)utes to the formation of a layer of nerve-fibres 



SECOND SERIES. — ZOOLOGY, VOL. VIII. H!) 



