344 THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM 



The Rhinencephalon or Olfactory Apparatus. — This is divided into a basal 

 portion and a pallial portion. The basal portion consists (i) in a ventral and 

 cranial evagination (pars anterior) formed mesial to the corpus striatum, which 

 is the anlage of the olfactory lobe and stalk (Fig. 328). 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 ex- 

 tends mesially along the lamina terminalis and laterally becomes continuous 

 with the tip of the temporal lobe (Figs. 323 and 328). This thickening consti- 

 tutes the anterior perforated space and the parolfactory area of the adult brain. 



The pallial portion of the rhinencephalon is termed the archipallium because 

 it forms the primitive wall of the cerebrum. It forms a median strip of the pallial 

 wall curving along the dorsal edge of the chorioidal fissure from the anterior 

 perforated space around to the tip of the temporal lobe, where it is again con- 

 nected with the basal portion of the rhinencephalon. The archipallium differ- 

 entiates into the hippocampus, 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 infolding of the hippocampus produces the hippocampal fissure. 



The Commissures of the Telencephalon. — The important commissures are 

 the corpus callosum, fornix and anterior commissure. The first is the great trans- 

 verse commissure of the neopallium or cerebral cortex, while the fornix and an- 

 terior commissure are connected with the archipallium of the rhinencephalon. 

 The commissures develop in relation 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 in stages between 80 and 150 mm. (Streeterin Keibeland Mall, 

 vol. 2). " It [the lamina terminalis] is distended dorsalward and antero-lateral- 

 ward 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 a space is formed, the fifth ventricle, or space of the septum pellucidum (Fig. 

 333 A, B). This space is bounded laterally by a portion of the median pallial 

 wall which remains thin and membranous, and constitutes the septum pellucidum 

 of the adult. 



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

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

 ing dorsal to the foramen of Monro (Fig. 333 A). In the cranial portion of the 

 lamina terminalis fibers are given off and received from the basal portion of the 



