378 



Special Vertebrate Organogenesis 



figuration, characteristic location and distri- 

 bution of cell columns within the unit, and 

 the like. 



Applying these tests, it has been found that 

 the consolidation of regional properties fol- 

 lows a definite time sequence: in general, 

 more anterior parts have a head start over 

 more posterior ones, and fixation along the 

 longitudinal axis seems to precede fixation 

 in the mediolateral direction. Presumptive 

 medulla oblongata of a urodele tail-bud em- 

 bryo transplanted in the place of the first 

 cord segments of another embryo develops 

 into a supernumerary medulla oblongata 

 (Detwiler, '25c). Even an isolated lateral 

 half of presumptive medulla grafted into a 

 lateral gap of spinal cord develops according 

 to its origin, quite incongruously for the 

 location, into half a medulla (Detwiler, '43a). 

 Yet, while anterior spinal levels at the same 

 stage show similar capacity for self-differen- 

 tiation (Maclean, '32), more posterior seg- 

 ments are still able in some measure to con- 

 form to site-specific determinative influences 

 (p. 387). 



All existing evidence leads to the conclu- 

 sion that, in the later neurula stage, alloca- 

 tion of differential developmental properties 

 within the CNS has already made notable 

 progress. The cord seems to lag slightly be- 

 hind the brain, but by the time different 

 cord segments have first become externally 

 distinguishable (in birds), each one after 

 transfer to another region develops essen- 

 tially as if it were still in the original place, 

 mosaic-fashion (B. Wenger, '51). The region- 

 ally specific "developmental properties" 

 contain the whole ground plan for all sub- 

 sequent transformations, that is, they deter- 

 mine pattern and basic rate of proliferation, 

 specific differentiation of cell types, migra- 

 tion, establishment of specific mutual group 

 relations, patterns of cell destruction, and 

 the physiological mechanisms underlying 

 functional activity and coordination. Thus 

 segmental organization in the chick embryo, 

 for instance, which begins to emerge on the 

 fifth day of incubation, shows up (Fig. 141) 

 in abundant cell degeneration in the cervi- 

 cal segments, segregation of a large lateral 

 motor column in the brachial segments, ab- 

 sence of a lateral column and presence of 

 a characteristic column of preganglionic 

 autonomic cells (nucleus of Terni) at the 

 thoracic and sacral levels (Levi-Montalcini, 

 '50; Hamburger and Levi-Montalcini, '50). 

 In the amphibians, where functional tests 

 have been carried fvtrther, it includes such 

 properties as the ability of the limb seg- 



ments to engender coordinated limb move- 

 ments, in contrast to the lack of this faculty 

 in the trunk and head regions (see below, p. 

 387). 



Similar early differences of properties at 

 different levels of the main axis have been 

 demonstrated for the spinal ganglia in re- 

 gard to cell proliferation and regulatory 

 ability (amphibians: Lehman and Youngs, 

 '52) and incidence of cell degeneration 

 (chick: Hamburger and Levi-Montalcini, 

 '49). 



It must be stressed that even as the mosaic 

 of diverse parts of the CNS emerges, each 

 area at first seems to operate "field"-fashion, 

 that is, while its general fate is fixed, the 

 detailed course of its component elements 

 would still be variable. This is indicated by 

 such observations as the following. At a 

 time when the development of the medulla 

 oblongata has already become irrevocably 

 identified with the part of the tube lying 

 at the presumptive hindbrain level (see 

 above), one can still turn that piece around, 

 reversing its anteroposterior axis, and yet 

 see it yield a medulla of normal (unre- 

 versed) configuration (Detwiler, '43b). Cord 

 regions of the same stage after anteropos- 

 terior reversal regulate similarly (Detwiler, 

 '24a). 



However, the interpretation of these regu- 

 latory effects calls for some caution. While 

 shape and proliferation rates, which served 

 as criteria, may indeed be as adaptable as 

 indicated, this need not apply equally to 

 the segregation of the various qualitatively 

 specialized cell strains, which might follow 

 a totally different time pattern and whose 

 determination might be either precocious or 

 delayed relative to those other features. 



Experiments with Mauthner's cell (M- 

 cell) seem to be pertinent to this prob- 

 lem. Unilateral extirpation in a post-tail-bud 

 stage of the hindbrain at the prospective 

 M-cell level leads to permanent absence of 

 an M-cell on the operated side (Detwiler, 

 '33). Since heterotopic grafts of the same 

 area from a neurula yield supernumerary 

 M-cells (Piatt, '44; Stefanelli, '50)— and as- 

 suming that the history of the M-cell is 

 representative of that of other, less con- 

 spicuous, cell types — we must conclude that 

 even at this early stage a rather detailed 

 fixation of type characteristics has occurred 

 in the various precursor cells of the later 

 cerebral neuron strains. Repetition of the 

 experiments in earlier stages using the 

 M-cell as marker has narrowed the critical 

 period of determination to the late gastrula 



