346 THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM 



The thalamus and corpus striatum are separated by a deep groove until the 

 end of the third month (Fig. 347). As the structures enlarge, the groove between 

 them disappears and they form one continuous mass (Fig. 350). According to 

 some investigators, there is direct fusion between the two. 



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

 portion and a palhal portion. The basal portion consists: (1) 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. 346). This receives the olfactory 

 fibers and its cells give rise to olfactory tracts. The tubular stalk coimecting 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 (Fig. 346) . This thickening constitutes the anterior 

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



The palhal 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 ampliibia. Later, when the neopallium, or adult cortex, arises, the 

 archipalKum 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 connected with the basal portion of the rhinen- 

 cephalon. The archipalhum 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 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, smaller in size, are connected with the archipalhum 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 palhal walls dorsal and cranial to it, the 

 lamina terminahs thickens rapidly in stages between 80 and 150 mm. (C R) 

 (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 palhum." Between the curve of the corpus callo- 

 sum and the fornix the median palhal walls remain thin and membranous, and 

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



