THE EVOLUTION OF THE CEREBRAL HEMISPHERES. 327 



pallium from the anterior commissure to the hippocampal com- 

 missure. The fibers appear first in the front part of the hippo- 

 campal commissure, with the fibers of which they are mingled. 

 As the neopallial fibers in this position increase in number the 

 hippocampal fibers are displaced backward and the dorsal com- 

 missure comes to be formed of two parts, an anterior corpus 

 callosum and a posterior hippocampal commissure or psalterium. 



The further development of the mammalian forebrain consists 

 chiefly hi the enlargement and increasing complexity of the 

 neopallium, the enlargement of the corpus callosum and the shift- 

 ing of parts consequent on these changes. The enlargement of 

 the neopallium and corpus callosum results in the degeneration 

 of the hippocampus through the greater part of its length. The 

 facts of chief importance are to be seen in the mesial aspect of 

 the hemisphere of various mammals or in sagittal sections near 

 the median plane. In the monotreme and marsupial (Fig. 167) 

 the hippocampus begins just above and behind the olfactory 

 peduncle, where it forms the boundary between the general cortex 

 (neopallium) and the precommissural body, and extends back 

 above the foramen of Monro and forms the mesial margin of the 

 roof of the hemisphere. Some distance from its anterior end the 

 hippocampus crosses the dorsal border of the lamina terminalis, 

 to which it is attached by reason of the fact that its commissure 

 crosses through the border of the lamina. Here also, just above 

 and in front of the commissure the lamina terminalis joins the 

 membranous roof of the median ventricle, the angle marking the 

 position of the recessus neuroporicus in the embryo. In the 

 marsupial and simplest mammalian brains the hippocampal 

 commissure takes on the form of a compact lamina and the corpus 

 callosum extends forward from it, so that the two form an inverted 

 V-shaped figure whose caudal descending limb is the hippocampal 

 commissure. The two commissures cause a thickening of the 

 lamina terminalis as indicated in the accompanying diagram (Fig. 

 168, C). 



The fibers of the corpus callosum in assuming this position 

 have run transversely through the hippocampal formation and 

 have mingled with the hippocampal ends of all the longitudinal 



