MEDULLA OBLONGATA 155 



large masses of musculature by sensory excitation of any kind, that 

 is, reactions of total-pattern type as defined by Coghill. This is the 

 only kind of response which can be made by Amblystoma in the 

 earliest stages of development of motility ; and in the adult animal it 

 is clear, from both the structural organization of the nervous system 

 and the overt behavior, that the action system as a whole is pre- 

 dominantly of total-pattern type. One of the coarser features of the 

 structural mechanism of this integrated behavior is seen in the waj^ 

 the neurons of the second order act as collectors of excitations of 

 diverse modalities and widely distributed peripheral origin. 



Within this primordial apparatus of integrated mass action, as de- 

 velopment advances, local areas are differentiated with progressively 

 more restricted functional connections. This is manifested anatomi- 

 cally by contraction of the spread of the dendrites of the secondary 

 elements so as to have synaptic contact with a smaller number of 

 fascicles of root fibers. In midlarval stages some of these elements 

 spread their dendrites through the whole extent of the white sub- 

 stance of the sensory zone, thus having contact with terminals of all 

 fascicles of root fibers (fig. 9, no. 1). Some of these dendrites also 

 reach downward into the zone of correlation, the reticular formation 

 (fig. 9, no. 4). Other elements are in functional relation with a few or 

 only one of the root fascicles (fig. 9, nos. 2, 3). In the adult the 

 dendritic spread is not so wide, and a larger proportion of the neu- 

 rons are in synaptic relation with one or a few contiguous fascicles of 

 root fibers; yet even here there are few neurons of the second order 

 which may be activated by root fibers serving a single modality of 

 sense. 



There is, accordingly, as development advances a progressive re- 

 striction and local specialization of the central apparatus of analysis 

 of the manifold of sensory experience, and this is an important factor 

 in the acquisition of the local reflexes which are individuated within 

 the larger total patterns of behavior as this process has been de- 

 scribed by Coghill. Furthermore, this process of local specialization 

 of structure within the sensory field, as exhibited in ontogeny, has a 

 parallel in phylogenetic history, as illustrated, for instance, by com- 

 parison of the organization of this field of Amblystoma with that 

 of the frog, as described by Larsell ('34, p. 504), and with that of the 

 specialized fishes ('44&). 



In the tracts of fibers which ascend to higher levels of the brain 

 stem from this sensory field — the lemniscus systems as described 



