Nervous System 



387 



but to all those other properties on which 

 coordinated function depends. Since the mere 

 attachment of a limb to a foreign spinal or 

 cerebral region does not induce there the 

 differentiation of effective limb control, the 

 experiments also confii^m our conclusion that 

 modulation plays no constructive part in the 

 design of central coordination patterns. 



The prevalent tendency to base orderly 

 central function on precise patterns of neu- 

 ronal connections gains little support from 

 the experimental work in embryonic and 

 larval stages. Any sufficiently large fraction 

 of the limb level of the cord contains the full 

 coordinative machinery for a limb; in am- 

 phibians one-third of the normal segments 

 is sufficient (Detwiler and Carpenter, '29; 

 Detwiler and McKennon, '30; Weiss, '36). 

 Not only reduction but considerable morpho- 

 logical disarrangement may be inflicted 

 upon the spinal limb centers without abol- 

 ishing the essentially coordinated develop- 

 ment of their typical action systems. For in- 

 stance, lateral halves of the limb cord resti- 

 tuted after ablation by regeneration from 

 the opposite half (Detwiler, '47b; Holtzer, 

 '51), as well as limb segments grafted in 

 dorso-ventral inversion (Holtzer, '50), still 

 yield the typical limb coordination patterns. 

 Antero-posterior reversal of the early tail- 

 bud medulla (Detwiler, '51) or midbrain 

 (Detwiler, '48) likewise fail to impair func- 

 tional activity appreciably (Detwiler, '52). 



The existence of serial functional locali- 

 zation in the spinal cord raises the question 

 of the time sequence and manner of its 

 origin. Is it already inherent in the early 

 neural plate organization, or is it acquired 

 only in the course of subsequent morpho- 

 genesis as a result of intracentral segrega- 

 tions and inductive interactions? The ap- 

 propriate test is the standard one of hetero- 

 topic transplantation or isolation. Overt 

 morphological criteria, such as architecture, 

 growth rates, cell numbers, and the like, 

 indicate that some of the regional differences 

 are rather firmly laid down as early as the 

 neural fold stage (see above, p. 376). How- 

 ever, we do not know to what extent these 

 morphological features signify distinctive 

 functional properties. Only a combination 

 of morphological and functional tests can 

 tell. Of the few thus far made, tlie follow- 

 ing are pertinent. 



When the prospective limb segments of the 

 cord of a urodele embryo in the neiu-ula 

 stage are replaced by a corresponding length 

 of trunk cord, which would normally have 

 remained smaller and incapable of control- 



ling a limb, the grafted piece acquires the 

 approximate size, as well as the functional 

 qualifications, of true limb segments (Det- 

 wiler, '23). Conversely, prospective limb 

 segments shifted to the trunk region remain 

 undersized and fail to develop the action 

 systems for limb control (Moyer, '43). Since 

 this positional adaptation appears prior to 

 the development of the limb, it must be 

 ascribed to intracentral regulatory inter- 

 actions, rather than to peripheral influences. 

 If trunk segments are grafted to the brachial 

 region in the later tail-bud stage, however, 

 the adjustment is incomplete and limb func- 

 tion remains defective (Table 6 in Detwiler, 

 '36b). Evidently, the spinal action systems 

 become fixed during that period. 



The spinal region anterior to the limb cen- 

 ters, when tested at the comparable stage 

 as before, shows less plasticity and tends to 

 "self-differentiate" in disharmony with its 

 new site (Detwiler, '25a). In its turn, how- 

 ever, it causes certain conforming modifica- 

 tions in the adjacent posterior host segments 

 (judged by their enlargement, as functional 

 tests have not been carried out). This ten- 

 dency toward self-differentiation followed by 

 some "inductive" influence spreading caudad 

 is even more marked when brain parts are 

 transplanted to more posterior sites, e.g., a 

 supernumerary medulla oblongata in the 

 place of anterior spinal cord (Detwiler, 

 '25c). 



It thus appears that functional localiza- 

 tion along the spinal axis continues into post- 

 neurulation stages, proceeding in antero- 

 posterior sequence. Whether the determina- 

 tion of regional cell number, the "baseline" 

 of our earlier discussion, is effected by a 

 separate agency from that determining func- 

 tional pattern is uncertain. If it were, some 

 sort of transneuronal stimulation such as 

 that noted above (p. 383) at the junction 

 of optic fibers and midbrain centers might 

 be invoked (Detwiler, '36b; see, however, 

 the objections raised on p. 375 regarding this 

 hypothesis). There are so few facts to go 

 by that speculation has free rein. What 

 is needed is more critical analytical research. 



Parenthetically, we may point to the po- 

 tential usefulness of heteroplastic transplan- 

 tation in the further exploration of this 

 field. Since exchange of parts between dif- 

 ferent species is feasible during the younger 

 stages of lower vertebrates, unique test com- 

 binations could readily be produced. Ex- 

 change of parts of essentially similar func- 

 tion would, of course, be less instructive 

 than exchange of those with regard to which 



