Herrick, Callosuvi and Hippocampus in Loivcr Brains. i8i 



which result in two curvatures in opposite senses, together 

 forming a " reverse curve." The primitive margin or point of 

 connection with the plexus remains the margin of the mantle in 

 spite of the accumulation of the fimbria or deeper fibres from 

 this region to produce a spurious marginal band. This relation 

 is rarely seen so clearly in adults as in Fig. 4, Plate IX, for 

 example, where, in spite of adhesions to the inner aspect of the 

 " cornu ammonis " for a distance, the thinned outer wall passes 

 ventrad to unite with the projection from the pyriform. Far- 

 ther cephalad this same thin wall is continued into the plexiform 

 tela uniting with the thalamus. In Figs. 2-3, Plate XXVII, 

 there is a very instructive illustration of the position of this 

 thinned part of the pallium where the free margins of the gyrus 

 dentatus are connected by it in the same longitudinal section 

 (x). Of course in passing ventrad the tela would be traced into 

 the plexus before uniting with the thalamus. Passing dorsad 

 the two parts of the dentatus unite, as in Fig. i. As to the 

 great histological distinctions which are claimed as differentiating 

 the gyrus dentatus from other cortical regions, we still claim 

 that many of them result from the tortions in form which throw 

 various parts into different planes and do not permit a direct 

 camparison in a given section with the histological structures of 

 the adjacent gyri. In the embryos just mentioned the gray 

 matter of the gyrus dentatus passes into that of the cornu am- 

 monis (gyrus fornicatus) without appreciable interruption and 

 the difference between the two sets of cells is no greater than 

 that between the last mentioned and the part of the mesal cor- 

 tex just dorsad to it. The cells of the cornu ammonis are 

 pressed into a close narrow band and are larger than those of 

 other cortical regions, but this seems as valid a reason for refus- 

 ing to regard the ammon's horn as cortex (as no one does) as 

 does the small size and oblique axial position and large granular 

 admixture of the gyrus dentatus. 



