346 THE MORPHOGENESIS OF THE CENTRAL ' NERVOUS SYSTEM 



The Rhinencephalon. The olfactory apparatus is divided into a 

 basal portion and a pallial portion. The basal portion consists: (i) of a 

 ventral and cranial evagination (pars anterior) , formed mesial to the cor- 

 pus striatum, which is the anlage of the olfactory lobe and stalk (Fig. 346). 

 This receives the olfactory fibers and its cells give rise to olfactory tracts. 

 The tubular stalk connecting the olfactory lobe with the cerebrum loses 

 its lumen. (2) Caudal to the anlage of the olfactory lobe a thickening of 

 the brain wall develops (pars posterior) which extends mesially along the 

 lamina terminalis and laterally becomes continuous with the tip of the 

 temporal lobe (Fig. 346). This thickening constitutes the anterior per- 

 forated space and the parolfactory area, of the adult brain (Fig. 356). 



The pallial portion of the rhinencephalon is termed the archipallium 

 because it forms the entire primitive wall of the cerebrum, a condition 

 which is permanent in fishes and amphibia. Later, when the neopallium, 

 or adult cortex, arises, the archipallium forms a median strip of the pallial 

 wall curving along the dorsal edge of the chorioidal fissure from the an- 

 terior perforated space around to the tip of the temporal lobe, where it is 

 again connected with the basal portion of the rhinencephalon. The 

 archipallium differentiates into the hippocampus (Figs. 345 and 349), a 

 portion of the gyrus hippocampi, and into the 'gyrus dentatus. It resembles 

 the rest of the cerebral cortex in the arrangement of its cells. The infold- 

 ing of the hippocampus produces the hippocampal fissure. 



Commissures of the Telencephalon. The important commissures 

 are the corpus callosum, fornix, and anterior commissure. The first is the 

 great transverse commissure of the neopallium, or cerebral cortex, while 

 the fornix and anterior commissure, smaller in size, are connected with the 

 archipallium of the rhinencephalpn. The commissures develop in rela- 

 tion to the lamina terminalis, crossing partly in its wall and partly in 

 fused adjacent portions of the median pallial walls. Owing to the fusion 

 of the pallial walls dorsal and cranial to it, the lamina terminalis thickens 

 rapidly during the fourth and fifth month (Streeter). "It [the lamina 

 terminalis] is distended dorsalward and antero-lateralward through the 

 growth of the corpus callosum, the shape of which is determined by the 

 expanding pallium." Between the curve of the corpus callosum and 

 the fornix, the median pallial walls remain thin and membranous, and 

 constitute the septum pellucidum of the adult. The walls of this sep- 

 tum enclose a cavity, the so-called fifth ventricle, or space of the septum 

 pellucidum (Fig. 351). 



The fornix takes its origin early, chiefly from cells in the hippocampus. 

 The fibers course along the chorioidal side of the hippocampus cranially, 

 passing dorsal to the foramen of Monro (Fig. 351 A). In the cranial 

 portion of the lamina terminalis, fibers are both given off to the basal 



