1 82 Journal of Comparative Neurology. 



EXPLANATION OF PLATES. 



PLATE IX. 



Figs. 1-4. Transverse sections of the fore-brain of pouch-embryos of the 

 opossum. For comparison with Plate XXV^IL Fig. i. shows the cephalic part 

 of the hippocampus. Fig. 4, shows how the gyrus dentatus is connected by a 

 non-cellular wall with the ventral mantle so that the ventricle theoretically 

 should be extended to the mesal terminus of the dentatus. 



Fig. 5. Transection of fore-brain of Menobranchns, to illustrate the fact, 

 discovered by Osborn, that the dorsal commissure is separated by layers of 

 ependyma from the ventral or pre-commissure. 



Fig. 6. Transection of the fore-brain of the alligator to illustrate the posi- 

 tion of the callosum and precommissure in the front of the terma. 



PLATE XXVn. 



Figs. i-j. A selection from a series of horizontal sections from the fore- 

 brain of pouch embryos of opossum to illustrate the relations of the dorsal com- 

 missures and hippocampus. Fig. i, is at the most dorsal level of the gyrus den- 

 tatus and illustrates the continuity of the cornu ammonis (fornicate gyre) with 

 the remainder of the dorsal cortex and the nature of the influences which have 

 transposed the relations of the gray matter. Figs. 2, 3 illustrate the continuity 

 of the mesal margin of the gyrus dentatus, though it is partially devoid of cells 

 (transformed into a tela.) In Figs. 3, 6 the hippocampal commissure and fim- 

 bria are clearly illustrated, while in Fig. 7, the callosum and strise Lancisii ap- 

 pear. 



Fig. 8. Longitudinal section from a similar brain showing the separation 

 between the callosum and hippocampal commissure. 



