SEPTUM, HIPPOCAMPUS, PALLIAL COMMISSURES 415 



the following: (a) The olfactory placode retards the closing of 

 the neural tube, causing the formation of the anterior neuropore. 

 The neuroporic recess, seen in most vertebrate brains, marks the 

 dorsal or caudal border of this neuropore. (b) Early in verte- 

 brate history the somatic sensory nerve of the telencephalic 

 segment is greatly reduced or disappears while the olfactory nerve 

 is very large in lower vertebrates. No motor nerve is present in 

 this segment, (c) The olfactory fibers enter the visceral sensory 

 column in the telencephalon and cause a great hypertrophy of 

 this column, which rises up in the brain wall and pushes the 

 somatic sensory column to the lateral surface ('11 a, p. 41; '11 b, 

 p. 497, 513-517). (d) The hypertrophy of the visceral sensory 

 columns together with the slight growth of ventral columns near 

 the optic chiasma has produced a forebrain flexure such that the 

 visceral sensory column takes the form of a letter U. The basal 

 limb of the U is occupied by secondary olfactory centers, its'dorsal 

 limb adjacent to the diencephalon by the olfacto-gustatory cor- 

 relation center (-'12 b, p. 369). The formatio olfactoria is sit- 

 uated in the base of the U and receives the olfactory nerve. The 

 space between the limbs of the U is filled by somatic sensory area. 

 (e) The evagination of the hemisphere begins first at the formatio 

 olfactoria, involves gradually the olfactory bulb and later the 

 olfacto-gustatory correlation center and finally the somatic sen- 

 sory area ('11 a, p. 42; '12 b, p. 363). The evagination does not 

 modify the fundamental relations, (f) The absence of a primar}^ 

 somatic sensory nerve in the telencephalic segment left the somatic 

 sensory column free to serve correlating functions for cutaneous, 

 kinaesthetic, visual and other somatic impulses. This has been 

 the controlling factor in the development of the general cortex 

 ('10 b). The assumption of terrestrial life has led to the rapid 

 development of this somatic area, and its expansion has pushed 

 the secondary olfactory centers and olfacto-gustatory center into 

 the positions occupied in mammals by the pyriform lobe and the 

 hippocampal formation respectively, (g) Interrelations between 

 the somatic area and the several regions of the visceral column 

 have resulted in the development of special centers; from the 

 larger part of the olfacto-gustatory center a hippocampus; from 



THE JOURNAL OF COMPARATIVE NEUROLOQT, VOL. 23, NO. 5 



