84 THE BRAIN OF THE TIGER SALAMANDER 



has physiological significance because the acute problems of subsist- 

 ence involve adjustments to surroundings. The primitive motor re- 

 sponses are mass movements, but the surface of the body is exposed 

 to a manifold of diverse stimuli which must be analyzed, and, accord- 

 ingly, diverse organs of sense were locally differentiated. The course 

 of this differentiation followed this rule: from the generalized and 

 equipotential to the special and local. Thus in some primitive eye- 

 less forms the entire skin is sensitive to light, in others only the most 

 commonly exposed surfaces of it; and in the leech, Clepsine, Whit- 

 man ('92) found that a single animal exhibited all transitions from a 

 series of segmentally arranged sense organs of generalized function in 

 the posterior part of the body to well-formed eyes at the anterior end. 



As I have elsewhere pointed out ('29), in the most simply organ- 

 ized vertebrates, the hagfishes, as described by Jansen ('30) and 

 Conel ('29, '31), the brain is organized around two dominant sensory 

 systems — olfactory and cutaneous — and the other special senses are 

 in various stages of arrest or degeneration in correlation with a semi- 

 parasitic habit. Without eyes, jaws, or limbs, the visible behavior is 

 reduced to a simple system of mass movements. Within the brain 

 there is little local differentiation except for the primary sensory and 

 motor fields directly connected with the peripheral end-organs, and 

 yet this brain is the adjusting mechanism of a very rigid system of 

 simple movements. 



Larval Amblystoma is similar, though here the specialization of 

 tissue is further advanced, and there is progressive advancement in 

 representatives of later phylogenetic stages. The principle of progres- 

 sive transformation from the general and dispersed to the specific and 

 local applies throughout phylogenetic development; it is a general 

 principle of embryogenesis (Weiss, '39, p. 288) ; it is clearly exempli- 

 fied in human development, as has been demonstrated on the physi- 

 ologic side by Hooker ('44) and on the structural side by Humphrey 

 ('44, p. 39); and it appears in the course of conditioning reflexes 

 (CoghiU, '30). 



In the phylogenetic history of vertebrates the basic pattern of 

 sensory equipment was apparently laid down very early, with no 

 radical changes except at the transition from aquatic to terrestrial 

 life. And at this period of transition from fish to tetrapod the neuro- 

 motor apparatus experienced even more radical transformation, with 

 elaboration of local reflexes which supplement and largely replace the 

 more primitive mass movements. 



In amphibian development this history is recapitulated in the long 



