SALAMANDERS AND THEIR BRAINS 17 



advanced, it is in directions that point clearly toward the mammalian 

 arrangement. 



Amphibian eggs and larvae are readily accessible to observation 

 and experiment; they are easily reared; they tolerate experimental 

 operations unusually well; and, in addition, the amphibian neuro- 

 muscular system begins to respond to stimulation at a very early age, 

 so that successive stages in maturation of the mechanism are docu- 

 mented by changes in visible overt movement. The adult structure is 

 instructive; and, when the embryological development of this struc- 

 ture is compared with that of higher brains and with the sequence of 

 maturation of patterns of behavior, basic principles of nervous or- 

 ganization are revealed that can be secured in no other way. In the 

 absence of differentiated cerebral cortex, the intrinsic structure of the 

 stem is revealed. Experimental decortication of mammals yields 

 valuable information, but study of such mutilations cannot tell us all 

 that we need to know about the normal operations of the brain stem 

 and the reciprocal relationships between the stem and the cortex. 



In brief, the brains of urodele amphibians have advanced to a 

 grade of organization typical for all gnathostome vertebrates, Amblys- 

 toma being intermediate between the lowest and the highest species 

 of Amphibia. This brain may be used as a pattern or template, that 

 is, as a standard of reference in the study of all other vertebrate 

 brains, both lower and higher in the scale. 



