382 J. B. JOHNSTON 



zone reaching from the vicinity of the neuroporic recess around 

 the rostral end of the hemisphere into the lateral wall. This 

 cell-free area which the writer has called the zona limitans hippo- 

 campi, marks the limit between secondary olfactory centers below 

 and the olfacto-gustatory correlating center above, the primordium 

 hippocampi. 



In the comparison of the telencephalon of selachians with that 

 of reptiles and mammals the identification of this zona limitans 

 would be of the greatest convenience. It may be said at once 

 that this boundary line is not always easy to recognize. A cell- 

 free zone occurring in the medial wall of the hemisphere is not 

 necessarily homologous with the zona limitans hippocampi of 

 selachians. The line of demarcation between any two function- 

 ally differentiated nuclei often presents itself in the form of a 

 cell-free zone. It is therefore necessary to examine the relations 

 of the centers in question. In all vertebrates the secondary 

 olfactory centers receive olfactory tract fibers and send fibers 

 to the hypothalamus and to the nucleus habenulae. In sela- 

 chians, if we confine our attention to the medial wall of the 

 hemisphere, three kinds of fibers pass across the zona limitans 

 between the medial olfactory nucleus and the hippocampal pri- 

 mordium: (a) olfactory tract fibers, (b) fibers arising from the 

 cells of the medial olfactory nucleus and tuberculum olfactorium, 

 known as the tractus olfacto-corticalis septi, and (c) the fornix 

 columns which descend from the hippocampal primordium to 

 pass caudad. In selachians the primordium hippocampi presents 

 no arrangement of cells into layers. The medial olfactory nu- 

 cleus presents two poorly marked cell layers. The outer one is 

 continuous below with the more compact and definite cortical 

 layer of the tuberculum olfactorium. This nucleus is present 

 in reptiles and mammals but has received little attention from 

 previous authors. It is shown in Herrick's figures 46 and 66 

 where it is called nucleus medianus septi; but the nucleus so 

 named in figure 43 is the primordium hippocampi and the nucleus 

 so named in figure 47 is that part of the paraterminal body which 

 surrounds the recessus praeopticus. The deeper nucleus forms 

 a projection into the lateral ventricle and has been called nucleus 

 accumbens septi (Kappers and Theunissen) nucleus septi (linger) 



