SEPTUM, HIPPOCAMPUS, PALLIAL COMMISSURES 395 



beneath the corpus callosum, and above the latter is a rather 

 large indusium. The prunordium hippocampi is broad and is 

 bounded below by very large islands of small cells which take 

 a very deep stain. At this level the primordium hippocampi 

 includes a large mass of gray between the sub-callosal hippo- 

 campal cortex and the ventricle. It is separated from the nu- 

 cleus parolfactorius lateralis by an oblique zona limitans. The 

 nucleus lateralis appears to be merely the medial part of the 

 head of the caudate nucleus which does not extend into the 

 medial wall beyond the lower angle of the ventricle. This rela- 

 tion is more striking in the rat than in any other mammal stud- 

 ied. Fifteen sections caudal to the last figure the very broad 

 sub-callosal hippocampal cortex has become reduced to a small 

 band (fig. 60) but the primordium is still large and bears the 

 same relation to the nuclei parolfactorii medialis and lateralis 

 and to the islands of Calleja as in other forms described above. 

 Caudal to this (fig. 61) the cortical band merges into the pri- 

 mordium, which maintains the usual relations. At the level of 

 the neuroporic recess (fig. 62) all the relations are closely similar 

 to those in the rabbit. Caudal to the foramen the primordium 

 extends a short distance among the fimbria fibers and then is 

 continued as a band of cells flattened between the hippocampal 

 commissure and corpus callosum, accompanying the fornix supe- 

 rior as in the rabbit and bear. These cells eventually are lost 

 to view among the cells of the deep or ventricular layer of the 

 hippocampus. 



Relation of 'primordium hippocampi to true hippocampus caudal 

 to the foramen interventricular e. The fact has been mentioned 

 above that in the rabbit the hippocampus extends rostrad be- 

 neath the corpus callosum almost to the level of the foramen 

 and overlaps the primordium hippocampi. The commissure and 

 fornix are imbedded in a continuous mass of gray which has 

 reached the cortical stage of differentiation behind the commis- 

 sure, while in front it remains an undifferentiated primordium. 

 The same is true of the mole. Here the hippocampus proper 

 extends rostrad only a short dista^nce beneath the splenium (fig. 

 88) but the primordium continues caudad over the foramen 



