522 TEXT-BOOK OF EMBRYOLOGY. 



They are joined by fibers from the dorsal surface of the callosum (fornioc 

 longus), i.e., from the vestigial hippocampal formation, many of which also 

 descend in front of the anterior commissure to the rhinencephalon. The trian- 

 gular mesial area (septum pellucidum) included between callosum and fornix 

 probably represents an extended part of the lamina terminalis or "commis- 

 sure-bed," in which a cavity is formed, the so-called fifth ventricle and ventricle 

 of Verga. A remnant of the hippocampal formation at the anterior end of 

 the callosum is represented by the gyms subcallosus (Fig. 445). 



THE NEOPALLIUM. 



The hippocampal or cornu ammonis formation and preterminal area 

 represent the older part of the pallium (archipallium) comp. pp. 438 and 439. 

 This part of the pallium is olfactory in character, being mainly a higher center 

 for the reception of secondary and tertiary olfactory tracts. In its extension 

 backward and partial obliteration by the corpus callosum, its embryologic 

 presents a striking similarity to its phylogenetic development (compare p. 438). 

 The rest of the pallial hemispheres (neopallium) are occupied by the non- 

 olfactory higher centers. 



The further growth of the neopallial hemispheres leads to their extension 

 backward, overlapping the caudal portions of the brain tube. In the course 

 of this extension the occipital lobe and its cavity, the posterior horn of the lateral 

 ventricle, are formed. The growth of various portions of the hemisphere sur- 

 face is unequal, producing folds (convolutions) and fissures. This folding 

 may be partly due to growth in a confined space, but especially important is 

 the relation between gray and white matter. The gray matter, containing not 

 only fibers but also neurone bodies, remains spread out in a comparatively thin 

 layer, probably to accommodate associative connections. The white matter, on 

 the other hand, increases in thickness. This leads to a folding of the outer 

 layer. The position of these folds is probably partly determined by the local 

 histological differentiation and growth of various cortical areas (p. 527). 

 Only some of the earliest and most important of these folds will be mentioned 

 here. 



It has been seen (p. 509) that early in the development of the pallium a 

 shallow depression appears on the external lateral surface of each hemisphere, 

 the fossa Sylvii (Fig. 446). The bottom of this is the future insula. It is ex- 

 ternal to the corpus striatum and does not grow as rapidly as the parts bound- 

 ing it, which consequently overlap it, forming its opercula. These bounding 

 walls are formed by the fronto-parietal lobe on its upper side, by the temporal 

 on its lower, and by the orbital on its anterior. The temporal and fronto- 

 parietal opercula begin about the end of the fifth month, the temporal at first 



