REFLEX AND CONDITIONED ACTION 43 



mander larvae has been intensively studied by Coghill 

 (1909-1926), and I have devoted considerable atten- 

 tion to the central mechanisms of correlation em- 

 ployed in later developmental stages of these animals 

 (see the summary by Herrick and Coghill, 191 5). 



From all of these researches it seems probable that 

 functional differentiation in the phylogeny (as in the 

 ontogeny) began at the periphery; and here the 

 elaboration of functionally specific end-organs and 

 conduction paths advanced much more rapidly on 

 the sensory side than on the motor side of the reflex 

 circuits. 



This does not mean that the sensory paths become 

 functional in embryonic development earlier than the 

 motor paths. In fact, in the development of the spinal 

 cord of Amblystoma larvae the converse is true, as 

 Coghill (1913) has shown. But it does imply that in 

 the progress of differentiation the sense organs are 

 structurally adapted to respond in a selective way 

 to a great variety of external stimuli at a stage when 

 the motor apparatus is so simply organized that there 

 is possible but little variety of modes of response to 

 these excitations. 



At an early larval stage when all of the sense 

 organs and sensory components of the peripheral 

 nerves are differentiated substantially as in the adult, 

 the motor mechanisms of the spinal cord and nerves 

 may show very little evidence of capacity for diversi- 

 fied response, simple swimming movements toward 



