Allen, Association in the Guinea Pig. 355 



and the lateral geniculate body (Corpus geniculatum laterale, 

 Cg) appear as prominent features. The principal fiber tracts 

 are already medullated. 



As in other rodents in which the olfactory apparatus is well 

 developed, the Ammon's horn (Cornu Ammonis, CA^ is a con- 

 spicuous structure. In the guinea pig at birth the fibers of the 

 Ammon's horn have only begun to be medullated in delicate 

 layers separated by large areas of cells and unmedullated fibers. 

 At the tip of the Ammon's horn the fimbria is already darkly 

 medullated. Dorsal to the Psalterium {Ps) and immediately 

 ventral to the Corpus callosum [Cc) is seen a group of fibers 

 belonging to the Forni.x longus {F I). 



Extending from near the median line dorsal to the fornix 

 around the cortex and ventrolateral to the lenticular nuclei is a 

 dark mass of fibers, the white substance of the hemispheres, 

 from which already numerous radial bundles emerge, passing 

 in the direction of the cortical cells. 



Not only are fibers to be seen radiating toward the surface, 

 but also there are man}- fibers in the deeper portions of the 

 cortex running parallel to the cortical surface. By using a high 

 power very fine fibers are visible in the zonal layer {Z). But 

 with this magnification no fibers are to be seen at the margin 

 of the cortex. In tlie white rat this system of fibers does not 

 begin to medullate until after the forty-second day. 



Development of the Cerebral Hemispheres. 



The drawings of the encephalon at thirty days and at ma- 

 turity show a gradual increase in number and complexity of 

 pathways formed by medullated fibers. The most remarkable 

 changes have been in the Ammon's horn {CA), in the Psal- 

 terium {Ps), in the internal capsule (O ). and in the white sub- 

 stance of the hemispheres. The optic tract appears well me- 

 dullated. The bundles of fibers radiating toward the cortical 

 surface from the white substance of the hemisphere become 

 more abundant between birth and maturity, and extend further 

 toward the surface. The fibers of the stratum zonale become 

 more evident and the layer becomes thicker. In general, there- 



[63] 



