392 



of the sensory nervous system had been 

 experimentally suppressed. The basic pat- 

 terns of motor coordination in such anesthetic 

 areas develop without major impairment, 

 and limbs lacking sensory innervation from 

 the beginning function coordinately without 

 sensory control having ever had a chance 

 to play a constructive part in the develop- 

 ment of the motor patterns (Weiss, '41a; 

 Yntema, '43; Detwiler, '47a). Since neither 

 learning nor patterns of sensory stimuli have 

 any basic part in the development of orderly 

 central functions, we must look to the auton- 

 omous processes of central development it- 

 self, outlined earlier in this chapter (pp. 376- 

 388), as the main source of coordination pat- 

 terns. The one fact that has been conclusively 

 established by experimental results is that 

 the central nervous system develops a finite 

 repertory of behavioral performances which 

 are pre-functional in origin and ready tt 

 be exhibited as soon as a proper effector ap- 

 paratus becomes available (Weiss, '41a). 



A clear distinction must be made between 

 the mere generation of a central discharge 

 and the pattern of its distribution (coordina- 

 tion). Contrary to a widespread belief, a 

 central discharge does not depend for its 

 generation upon afferent influx but can 

 originate within the centers. Many instances 

 of rhythmic automatisms of nerve centers 

 have been reported (see Bremer, '53) and 

 referred to underlying fluctuations in the 



Special Vertebrate Organogenesis 



metabolic and electric state of the nem^ons 

 in interaction with the humoral milieu. The 

 observation that any isolated and deranged 

 fragment of medulla or spinal cord will 

 permanently exhibit trains of spontaneous 

 rhythmic discharges (Weiss, '41b) suggests 

 that such activity is a basic property of pools 

 of neurons rather than a specialty of certain 

 centers only. The different rates of "spon- 

 taneity" in urodeles with removed or vari- 

 ously exchanged brain parts (Detwiler, '48) 

 may be indicative of differential pacemaker 

 loci in the central generator network. 



The sole purpose of these very sketchy 

 comments on behavioral development has 

 been to illustrate the crucial role the methods 

 of experimental embryology and morphology 

 are destined to play in arriving at objective 

 and exact information, and in dispelling 

 misinformation, concerning the principles of 

 behavior. 



CONCLUSIONS 



The only valid summary of this chapter 

 on neurogenesis is to say that, by the very 

 nature of the developmental process, it does 

 not lend itself to verbal summarization. Any 

 attempt to embrace such intricately compli- 

 cated events as those dealt with in this chap- 

 ter in a simple and glibly summarizing 

 formula would end up with either meaning- 

 less platitudes or fictitious oversimplifica- 



GENIC ENDOWMENT 



CAUSAL RELATIONS 



N 



NEUROGENESIS 



CELL STRAIN 



SPECIATION ICILIATION 



CENTRAL 

 ASSOCIATIONS 



Fig. 144. 



