266 THE BRAIN OF THE TIGER SALAMANDER 



Comparison of the amphibian cerebral hemisphere with the human 

 shows a common plan of organization, and in the amphibian brains 

 we find evidence of the beginning of differentiation of some mam- 

 malian structures at the earliest stages of their emergence from an 

 undifferentiated matrix. The formative agencies which are operating 

 to produce this local specialization are open to inspection and experi- 

 mental investigation. 



On the basis of position, internal structure, and connections the 

 following mammalian organs have been identified in the brain of 

 Amblystoma. First, the pallial field is distinguishable from the stem, 

 and within this field primordia of hippocampal and piriform cortical 

 areas are unmistakable. Some connections are suggestive of influ- 

 ences which may be precursors of neopallial differentiation, but these 

 are vague and uncertain. Most of the mid-dorsal pallial area is prob- 

 ably represented in higher brains at the margins of archipallial and 

 paleopallial cortex — such transitional cortex as the subicular and 

 perirhinal areas. 



In the subpallial part the lateral and medial walls of the hemi- 

 sphere are organized essentially as in mammals. Laterally, the strio- 

 amygdaloid complex is well defined, though its subdivisions are not 

 clearly separable. Of these, the amygdala is definitely organized, with 

 connections very similar to those of mammals. In the corpus striatum 

 the nucleus accumbens septi is present as in lower mammals, and 

 associated with it is an area which probably corresponds with the 

 head of the caudate nucleus. The remainder of the corpus striatum is 

 an undifferentiated lentiform nucleus, within which large and small 

 cells are mingled. The connections of these cells suggest that the 

 dorsal part of this area becomes the putamen and the ventral part 

 the globus pallidus (p. 96). 



On the medial side of the hemisphere the structure and connections 

 of the septum conform with the mammalian arrangement, and below 

 this is an undifferentiated area which gives rise in some of the fishes 

 and in mammals to the tuberculum olfactorium. 



THE OLFACTORY SYSTEM 



As outlined in chapter vii, the olfactory nerve and its connections 

 have played the dominant role in the morphogenesis of the cerebral 

 hemispheres of lower vertebrates. The brief summary of the structure 

 at the end of chapter iv is here supplemented by further description 

 of the distribution of the olfactory tracts. 



