416 J. B. JOHNSTON 



contiguous parts of the medial olfactory nucleus and somatic 

 area, the corpus striatum; and from contiguous parts of the 

 lateral olfactory nucleus and somatic area, the pyriform lobe. 

 The details of all these processes remain yet to be worked out. 



In the present paper we are concerned chiefly with the rela- 

 tions in the medial wall of the hemisphere and not with the general 

 cortex or the pyriform lobe. The main questions at issue are the 

 identification in reptiles and manmials of the basal olfactory 

 center (medial olfactory nucleus or area parolfactoria) and the 

 equivalent of the massive roof of the selachian telencephalon, the 

 boundary line between the two, and the position of the forebrain 

 commissures with relation to these two bodies. The result of 

 our study has been to show: 



1. That the neuroporic recess is situated just above the anterior 

 commissure and below the anterior pallial commissure and be- 

 tween the pillars of the fornix when those structures are present. 

 It is the recessus triangularis of Schwalbe and recessus inferior of 

 Elliot Smith. 



2. The boundary line between roof structures and area parol- 

 factoria is marked in reptiles and many mammals by a groove 

 running rostrad from the neuroporic recess in the ventricular sur- 

 face of the medial wall and by a zona limitans in the structure of 

 the wall. The zona limitans and ventricular sulcus are both the 

 expression of functional differentiation of adjacent centers and 

 indicate a rearrangement of the neurones in the gray matter ad- 

 jacent to the ventricle. 



3. The structures dorsal to this zona limitans include the hip- 

 pocampal formation proper and the septum pellucidum or its 

 equivalent in the brains of lower mammals and reptiles. These 

 structures are all develoj^ed out of the roof of the selachian brain, 

 called by the writer the primordium hippocampi. 



4. The structures ventral to this zona limitans include a nucleus 

 lateralis closely related to the head of the caudate nucleus, a 

 nucleus medialis on the medial surface, islands of Calleja and 

 the tuberculum olfactorium. 



5. The hippocampal formation gradually differentiates a cor- 

 tical layer in the medio-dorsal region of the hemisphere and, as 



