Nervous System 



379 



(in frogs). At that stage, while an explant 

 or transplant from the presumptive M-cell 

 region can already give rise to an M-cell 

 independently, another M-cell may develop 

 at the normal site from amidst the residual 

 neural cells that have closed the gap from 

 adjacent levels (Stefanelli, '50). This proves 

 that the factors localizing M-cell develop- 

 ment in the hindbrain had already been in 

 operation but were still enough active to 

 turn out a second set; furthermore, that 

 there were still cells in the surrounding 

 brain regions sufficiently labile in char- 

 acter to respond to redirection. These results 

 suggest that the character of cell strains is 

 fixed much earlier than are the numbers, 

 distribution and arrangement of their de- 

 scendent cells. 



Determination in the transverse direction, 

 that is, along the mediolateral axis of the 

 neural plate, is likewise a gradual process. 

 This can be concluded from the fact that 

 at stages at which transverse strips of neural 

 plate manifest definite "self-differentiation" 

 of regional character after displacement, 

 they still are capable of extensive regulation 

 in the lateral direction (Roach, '45). How- 

 ever, at the stage of the closure of the neural 

 folds, some further mosaic subdivision has 

 also developed in the mesiolateral direction 

 (which after folding appears as ventro- 

 dorsal direction) ; for in birds, localized 

 defects placed in the neural epithelium at 

 that stage result in cords in which the whole 

 radial sector that woidd normally arise from 

 and cover the destroyed patch is completely 

 missing (E. Wenger, '50). 



It is to be noted that these resvxlts prove 

 only the loss of capacity for regulative re- 

 distribution of tasks within the residual 

 fragment of nervous system itself; no re- 

 generation of the missing sectors had been 

 initiated. If regeneration, i.e., mobilization 

 of new cell material to replace missing 

 parts, can be activated, as has been possible 

 in amphibians, the regulative faculty ex- 

 tends into considerably later stages; again, 

 it is far wider in the lateral than in the 

 longitudinal direction. After the excision of 

 a lateral half of presumptive midbrain, 

 medulla or spinal cord segments from the 

 neural tube, increased proliferation and mi- 

 gration of cell masses across the midline 

 from the residvial intact half restore the 

 missing portion with remarkable morpho- 

 logical and, in earlier stages, also histologi- 

 cal, perfection (Detwiler, '44, '46b, '47b; 

 Harrison, '47; Holtzer, '51). 



A comparative view of all these results 



leads us to distinguish rather sharply be- 

 tween determination of individual cell fate, 

 strictly cell-wise, on the one hand, and 

 "determination" of a cell complex, on the 

 other, the latter connoting the imparting 

 of some frame of conditions to the cell group 

 which only in further consequence would 

 gradually fix the characters of the individual 

 component cells. In the case of the CNS, 

 this presents vis with the alternative that 

 either (a) the diverse cell types of the later 

 mantle are already preformed as such in a 

 corresponding variety of precursor cells in 

 the neural epithelium, or (b) the cells of 

 the neviral epithelium are still equipotential 

 and acquire their differential type character- 

 istics only through local influences of the 

 different mantle portions in which they 

 come to lie. 



There is not enough evidence on hand 

 to decide this alternative crucially one way 

 or the other. Except for some indirect mor- 

 phological and pathological indications 

 (Globus and Kuhlenbeck, '44), it has not 

 even been definitely settled whether the 

 dichotomy between glia (spongioblasts) and 

 nerve cells (neuroblasts) is already effected 

 in the germinal epithelium or whether 

 both are derived from common stem cells. 

 In the case of Mauthner's cell described 

 above one could assume, according to (a), 

 that during neurulation a particular cell of 

 the plate is endowed with the ability either 

 to turn into an M-cell itself or to undergo 

 an orderly seqiience of unequal divisions 

 eventuating in the segregation of one of the 

 descendents as an M-cell. A model for such 

 a process is known, for instance, in the pro- 

 duction of the mother cells of scales in 

 certain insects (Henke, '53). The occasional 

 occurrence of twin M-cells in haploid em- 

 bryos (Fankhauser, '52) could be taken as 

 a sign of the disturbance of the regular cell 

 lineage because of the vmdersized mass of 

 the haploid cells. According to (b), one 

 would assvime that a particular cell block 

 in the presumptive hindbrain woi;ld be 

 endowed with "inductive" activities that 

 would reach a rather sharp peak at a given 

 focus, and that the cell that happened to be 

 thus pin-pointed would thereby be singled 

 out to grow up into an M-cell. 



There is some suggestive, but meager, 

 evidence pointing to early cell type diver- 

 gence according to (a). In the experiments 

 on the repair of excised halves of spinal cord 

 from cell sources of the opposite half (see 

 above, this page), it was noted that when the 

 operation was performed in successively 



