SEPTUM, HIPPOCAMPUS, PALLIAL COMMISSURES 389 



callosum in mammals, the attention of the reader is called to 

 figures 12 to 16, representing sections of the brain of the turtle 

 rostral to the commissures. They are sufficiently described in 

 the explanation of the figures. 



In the mole, bat, rat, gopher, and rabbit the precallosal hippo- 

 campus approaches the ventral aspect of the genu and con- 

 tinues caudad beneath the corpus callosum to become lost in or 

 fused with the underlying septum pellucidum (undifferentiated 

 primordium hippocampi). This will appear more clearly in the 

 description of sagittal sections below. 



A.S is well known, the hippocampal formation also continues 

 caudad over the corpus callosum. Figures 42, 50, 58 and 66 

 show the hippocampal formation as it bends around the genu. 

 It is accompanied by precommissural fibers of the fornix system 

 some of which come from farther rostrad while some come up 

 through the septum pellucidum to course around the genu. Below 

 the hippocampal formation and occupying the whole thickness 

 of the wall is the primordium hippocampi. It is small in the 

 rabbit, very large in the mole and in the bat. Below this are 

 the nucleus parolfactorius medialis and nucleus parolfactorius 

 lateralis. Examination of sections farther forward as well as 

 those now under consideration, shows that the nucleus lateralis 

 is in direct continuity around the ventral angle of the ventricle 

 with the nucleus caudatus. The two bodies seem to have the 

 same structure and to form parts of one mass. This is a very 

 conspicuous fact in all the forms examined and has already been 

 noted by Unger and by Kappers. The boundary between the 

 lateral nucleus and the primordium hippocampi is marked by 

 a slight ventricular sulcus, which has been described above from 

 dissections, and a well marked cell-free zona limitans. This zona 

 limitans does not cross the medial wall directly, but at this level 

 appears as a semicircular line of division between the nucleus 

 parolfactorius lateralis and nucleus caudatus internally and the 

 superimposed tuberculum olfactorium externally. Just medial 

 to the zona is seen in most lower mammals either an incom- 

 plete and irregularly broken plate of darkly staining cells (Nissl 

 preparations) or a few isolated masses of such cells. These are 



