384 



Special Vertebrate Organogenesis 



In discussing nerve outgrowth, we have 

 referred to several provisions for the over-all 

 guidance of masses of motoneurons to mus- 

 cle masses as their appropriate destinations 

 (selective contact guidance, selective fascicu- 

 lation, etc.), as well as for the preclusion 

 of peripheral connections with inappropriate 

 kinds of tissues. It must be remembered, 

 however, that orderly motor function pre- 

 supposes that the whole muscle mass of a 

 region be not thrown into contraction indis- 

 criminately or all at once, but that at any 

 one moment, only definite selections of indi- 

 vidual muscles be made to contract in com- 

 binations yielding a "coordinated" move- 

 ment. Physiological concepts of coordination 

 are still rather controversial but they all 

 agree on one point, namely, that the co- 

 ordination of muscle contraction is the re- 

 sult of the selective excitation of the motor 

 ganglion cells connected with the muscle 

 fibers to be actuated. Thus, in order to effect 

 an orderly movement, the motor centers 

 must "know" precisely which ganglion cells 

 are hitched to what muscles. 



Theoretically, the way in which this 

 knowledge is acquired could be conceived 

 of about as follows: Either (a) there is a 

 predestined motor cell group for each indi- 

 vidual mviscle and some detailed mechanism 

 exists by which the axons of that cell group 

 are routed precisely to the matching muscle 

 to the exclusion of all other muscles, so that 

 the pattern of connections woidd be stereo- 

 typed for all individuals; or (b) lacking such 

 stereotvped connections, the centers would 

 "learn" about their snecific relations to the 

 periphery by "trial-and-error." actuating 

 muscles at first in random combinations and 

 then fixins: somehow those central linkages 

 that had incidentallv yielded useful move- 

 ments. Most past and current thinking about 

 coordination implies either one or the other 

 of these assumptions. Yet, both are contra- 

 dicted by the facts. As for (a), predestina- 

 tion of motor fibers for particular muscles 

 (rather than just muscles in general) is 

 ruled out not only by the normal variabilitv 

 in plexus formation, but above all by the 

 experimental proof that any motor fiber will 

 innervate equally readily any individual 

 muscle (see above, p. 363) and that, as will 

 be described presently, coordinated function 

 can be obtained even after deliberate ran- 

 domization of the peripheral pathwavs. And 

 as rearards (&), not only has the "trial-and- 

 error" period supposed to produce basic co- 

 ordination by gradual learning never been 



observed (see later, p. 391), but, as we shall 

 show below, the very crux of this thesis, 

 that the degree of utility to the individual of 

 an achieved movement is at all critical for 

 the development of coordination, has been 

 experimentally invalidated. Neither alterna- 

 tive being tenable, the solution came from a 

 third and unexpected direction. 



In condensed version, it is as follows: (1) 

 Each individual muscle has some consti- 

 tutional specificity by which it is distin- 

 guished (presumably in its finer protoplas- 

 mic chemistry) from all other muscles (ex- 

 cept homologous ones); (2) it imparts its 

 specificity to the motor nerve fiber to which 

 it has become attached, and through the 

 axon, to the ganglion cell; this progressive 

 specification of motoneurons by, and in con- 

 formance with, their individual terminations 

 has been termed "nerve modulation." It is 

 through this direct epigenetic backward pro- 

 jection of the mosaic of muscular specifici- 

 ties upon the population of motor ganglion 

 cells that the centers are informed as to 

 just where their communication lines ter- 

 minate. For fuller information, the reader 

 may be referred to earlier reviews (Weiss, 

 '36. '50b, '52a). Only the basic experiment 

 will be brieflv summarized here. 



"When a limb muscle of a larval am- 

 phibian is transplanted near a normal limb 

 and provided with innervation from a nerve 

 branch diverted from the normal limb 

 plexus — any limb nerve branch — the trans- 

 planted muscle is always found to contract 

 simultaneously with the muscle of the same 

 name in the normal limb (principle of "myo- 

 tvpic" fiTUction). With several supernumer- 

 ary muscles, the rule applies to each of 

 them separately, so that if a whole limb 

 with a full extra set of muscles is added, 

 their total activity duplicates the overt ac- 

 tions of the normal limb; or if transplant 

 and normal limb are of reverse asvmmetrv, 

 their movements mirror each other (Fig. 

 142). Thus, aranerlion cells of the limb level 

 of the cord that happen to innervate muscles 

 of the same name (i.e., of identical consti- 

 tutional specificity) become functionally 

 linked, even though the functional effects 

 of supernumerary muscles or limbs are use- 

 less, or outright detrimental, to the animals. 

 Since in these experiments the choice of 

 e-anfi:lion cells for the test mviscle was en- 

 tirelv a matter of chance or assignment 

 by the experimenter, it is evident that the 

 muscle must have conveved its name to the 

 central cell, and since this happens even in 



