298 C. JUDSON HERRICK 



and I find the same relations in half grown larvae. Coghill 

 ('16) has studied the development of each of these functional 

 systems in the youngest larvae. Here, as in the development of 

 the spinal nerves (Coghill, '14 and Herrick and Coghill, '15), the 

 elaboration of the pattern of the peripheral nerve components 

 is accompanied by striking changes in the orgnization of the 

 central nervous system. 



From these researches it is suggested that functional differ- 

 entiation in the phylogeny, as in the ontogeny, began at the 

 periphery; and here the elaboration of functionally specific end- 

 organs and conduction paths advances much more rapidly on the 

 sensory side than on the motor side of the reflex circuits.^ At 

 an early stage when all of the sense organs and afferent nerve 

 components are differentiated substantially as in the adult the 

 motor mechanisms of the spinal cord and nerves may show very 

 little evidence of capacity for diversified response, .simple swim- 

 ming movements toward or away from the source of the stimulus 

 being almost the only possible reactions. In other words, the 

 elaborately diversified receptor mechanisms converge into a 

 very simply organized final connnon path. Even in the half 

 grown larva the motor apparatus of the spinal cord seems to be 

 organized to serve chiefly total reactions of the simplest loco- 

 motor type. 



In the medulla oblongata of the half grown larva we can 

 follow some of the steps in the gradual emergence of functionally 

 specific primary sensory centers and their correlation tracts 

 from a relatively equipotential receptive apparatus which origin- 

 ally discharged directly and without functional analysis of 

 different sense qualities into the motor final common path (Her- 

 rick, '14 a). In these larvae various lemniscus systems w^hich 

 discharge specific kinds of sensory nervous impulses upward 



^ This does not mean that the sensory paths become functional earlier than 

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

 larvae the converse is true, as Coghill ('13) 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. 



