92 THE BRAIN OF THE TIGER SALAMANDER 



As already pointed out, structural differentiation of cortex is in- 

 cipient in the hippocampal sector of the pallium in all Amphibia; 

 more clearly defined primordia of cortex are seen in adult lungfishes 

 and in embryonic stages of some other fishes (Rudebeck, '45) ; but 

 well-differentiated cortex of typical structure first appears in reptiles. 

 The problem set is : What morphogenic agencies were operative dur- 

 ing the emergence of cortex from a noncorticated matrix? 



Early in the attack upon this problem it became evident that the 

 key factors were to be sought, not in the pallial field, but in its 

 environs. What comes into this field and what goes out of it at suc- 

 cessive stages of morphogenesis and how are these factors related 

 one to another in both subpallial and pallial territory.^ This requires 

 in the upshot a histological analysis of the entire nervous system 

 directed toward the physiological interpretation of all visible struc- 

 ture. In my Brains of Rats and Men ('26) the available evidence 

 regarding the origin of cerebral cortex was surveyed. Since that time 

 additional evidence has been recorded, and in the present work the 

 results of a renewed examination are summarized. The problems 

 centering in corticogenesis have not been solved, but some progress 

 has been made. The conclusions reached can, at best, be only tenta- 

 tive, pending physiological control; and for such experiments exact 

 information about the anatomical arrangements of parts is indis- 

 pensable. 



MORPHOGENESIS OF THE CEREBRAL HEMISPHERES 



There is reason to believe that in the early ancestors of vertebrates 

 the central nervous system was a simple tubular structure com- 

 parable with that of living Amphioxus. Accompanying enlargement 

 of the dominant sense organs — nose and eye — the anterior part of 

 this tube was expanded in four places, which became cerebral hemi- 

 spheres, hypothalamus, epithalamus, and tectum opticum. In most 

 lower vertebrates the olfactory system is very large and has played 

 a dominant role in the earlier stages of the morphogenesis of the 

 hemispheres. In fishes this differentiation took a wide variety of 

 forms, some of which were surveyed in two papers ('21, '22a). These 

 diverse forms and patterns of internal structure were shaped in 

 adaptation to various modes of life which employed different equip- 

 ment of sensory and motor organs. 



It has been mentioned that the more sluggish fishes, and especially 

 mudfishes living in stagnant water, have enormously enlarged and 



