SEPTUM, HIPPOCAMPUS, PALLIAL COMMISSURES 413 



of the hippocampal formation and in it the hippocampal com- 

 missure collects before it descends in the lateral wall behind the 

 foramen. With the exception of the behavior of the hippocampal 

 commissure the relations in the frog agree with those in the 

 turtle. Behind the foramen it is not difficult to see that this 

 body corresponds to the small primordium hippocampi in this 

 position in the turtle and the sulcus along (beneath) which the 

 fimbria runs is the fimbrio-dentate sulcus. Rostral to the fora- 

 men in the frog there is wanting the sulcus which separates the 

 primordium hippocampi from the area parolfactoria in reptiles 

 and mammals. Gaupp ('97, figs. 26, 28, 29) describes and figures 

 a narrow cell-free zone between this nucleus and the cells below. 

 He calls this column ''ganglion septi," but it is entirely different 

 from the nucleus septi of linger, Kappers and others. The en- 

 tire lower half of the medial wall has been assigned by most 

 authors to the paraterminal body and it has been assumed that 

 the paraterminal body extends up over the foramen. 



Now this assumption is one of the most inexplicable things in 

 the literature of forebrain morphology. It is quite possible to 

 understand how the great growth and arching dorsad of the 

 corpus callosum is mammals results in stretching up the com- 

 missure bed as set forth by Elliot Smith. And so long as it was 

 thought that the commissure bed belonged below the neuroporic 

 recess and was related to the lamina terminalis there was no 

 escape from the logic of the argument so far as applied to mam- 

 mals, that the paraterminal body had been stretched up into 

 the supraforaminal position as the septum. But in reptiles and 

 amphibians there is no such voluminous corpus callosum. There 

 is therefore absolutely no motive for the extension of the para- 

 terminal body up over the foramen. In amphibians there is 

 not even a hippocampal commissure extending up over the fora- 

 men from in front. Moreover it is known that in reptiles at 

 least the bed of the pallial commissures is supraneuroporic in 

 origin. The supposition that a part of the paraterminal body 

 has migrated into the caudal part of the hemisphere above the 

 foramen is not only not supported by evidence but is quite 

 unthinkable in view of all the known facts. 



