386 



Special Vertebrate Organogenesis 



iheir specificities to other, more proximal 

 neurons (Weiss, '41a; Sperry, '51). None of 

 this, however, touches on the problem of 

 coordination and its origin as such; that is, 

 on the mechanisms by which the individual 

 units are actuated in such orderly groupings 

 and sequences that their total effect yields 

 an integrated movement, such as walking, 

 swimming, feeding, etc. As the physiological 

 study of the neural elements has far out- 

 distanced the understanding of their group 

 behavior, we are still without a concept of 

 coordination that could claim cogency or 

 general applicability. This matter would be 

 of no particular concern to us in the present 

 context but for two reasons: first, no con- 

 sideration of neurogenesis would be complete 

 unless it included some account of the de- 

 velopment of the "integrated activity" of 

 behavior; with this we shall deal briefly 

 later. And second, much light can be shed 

 on the nature of coordination from a study 

 of its ontogeny; for this, the experiments 

 just reported offer a relevant example. 



The experiments have shown that not only 

 does an extra set of muscles operate in the 

 correct combinations and time sequences re- 

 quired for normal coordination (as expressed 

 by the near-by normal limb), but a single 

 original set of muscles, when scrambled or 

 otherwise abnormally arranged and inner- 

 vated, also operates in the same stereotyped 

 order; that is, each muscle as an individual 

 contracts at such time and in such strength 

 as would be called for in the particular 

 movement of a normal limb (Weiss, '41a), 

 regardless of the fact that, owing to the 

 anatomical disarrangement, this blind execu- 

 tion of sequences designed for normal ar- 

 rangement results in a wholly senseless 

 performance. Thus, an amphibian provided 

 with limbs of reverse symmetry (by ex- 

 changing right and left limbs) executes all 

 movements in reverse, e.g., walks backwards 

 whenever it is due to advance, and vice 

 versa, all its life (Weiss, '37b). Since this 

 occurs likewise in animals in which the 

 limbs had been reversed as buds and which 

 therefore had never experienced the use of 

 normal limbs, it is plain that the basic 

 coordinating mechanisms, which call the 

 different muscles for a given movement into 

 operation in the proper selection and se- 

 quence, are intrinsic and stereotyped prod- 

 ucts of CNS development and operate blindly 

 regardless of the effectiveness or inefficacy 

 of the resulting movements. These central 

 mechanisms in which the various coordina- 

 tion patterns are preformed might be called. 



with a non-committal term, "central action 

 systems." They deal not with the muscles 

 as such, but with the ganglion cells modu- 

 lated by the latter. Modulation merely sets 

 the muscles into the proper response rela- 

 tions with the central action systems, but 

 it does not govern their construction. For- 

 mally, the relation between central action 

 systems and the modulated receptor and 

 effector neurons resembles communication 

 by "resonance." 



In conclusion, the response relations 

 within the CNS are now recognized to be 

 ruled by qualitative specificities of great 

 subtlety, far beyond morphological detec- 

 tion, and not simply by geometrical relations 

 of otherwise equivalent units. 



DEVELOPMENT OF CENTRAL ACTION 

 SYSTEMS 



The realization that in order to call forth, 

 for instance, a coordinated elbow movement, 

 the CNS must have developed the specific 

 means to excite ganglion cells modulated 

 by elbow muscles, presents us with a prac- 

 tical test of the presence or absence of spe- 

 cific action systems in a given central sector: 

 a muscle group transplanted to the sector 

 will not operate unless the center contains 

 the matching set of specific activators. Ap- 

 plying this test, it was found that coordi- 

 nated limb activities are engendered only 

 within the normal limb segments of the 

 cord (Detwiler and Carpenter, '29). Even 

 a completely isolated brachial cord section 

 can yield coordinated activities in a limb 

 innervated by it, whereas an isolated piece 

 of trunk cord in otherwise identical circum- 

 stances cannot (Rogers, '34). The function 

 of limbs innervated by trunk nerves alone 

 remains abortive. Similarly, limbs trans- 

 planted to the head and innervated by 

 cranial nerves, while twitching in associa- 

 tion with head muscles, never exhibit or- 

 derly independent movements (Detwiler, 

 '30b); moreover, such movements as are ob- 

 served are attributable chiefly to local eye, 

 gill or gular muscles that have attached 

 themselves to the skeleton of the grafted 

 limb, with the limb muscles themselves be- 

 ing essentially uninvolved (Weiss, '36; Piatt, 

 '41). We learn from these results that the 

 regional differences within the cord, some 

 of whose quantitative expressions we have 

 previously encountered (under Regional and 

 Cytological Differentiation, p. 376), are 

 really much more profound, pertaining not 

 only to numbers and configurations of cells 



