Growth and Differentiation in the Nervous System 283 



A comparison of the effects called forth by extirpation of the limb 

 primordia or by implantation of additional limb buds showed that in 

 both instances all developmental processes of sensory nerve cells 

 seemed to be affected — the mitotic activity, the initial differentiation 

 of sensory nerve cells, as well as their further growth ( 32 ) . 



Two hypotheses were advanced as to the mechanism of this control. 

 Either (1) the peripheral field of innervation controls the growth 

 processes of the associated nerve centers by releasing into the circula- 

 tion some agent which selectively promotes the growth and differen- 

 tiation of nerve cells tributary to that field, or (2 1 the periphery 

 affects only those nerve cells which establish contact through their 

 nerve fibers with the field of innervation. Both alternatives failed to 

 explain satisfactorily all the effects evoked in the sensory ganglia by 

 extirpation or transplantation of an additional limb. The hypothesis 

 listed at (1) was in conflict with the observation that an increase or a 

 decrease of the peripheral field of innervation affects only the sensory 

 and motor centers which supply nerves to that area. The other alterna-. 

 tive, which considers the nerves as mediators between the end organs 

 and the associated nerve centers, agrees better with these observations. 

 It does not, however, explain other aspects of the growth response of 

 the sensory ganglia to changes inflicted in their peripheral field of 

 innervation. It was in fact found that the mitotic activity in these 

 ganglia increases or decreases according to the extension of the area 

 they innervate. Since dividing cells are obviously deprived of nerve 

 fibers, these results cannot he accounted for as a direct effect of the 

 periphery on individual nerve cells. The experiments to be reported 

 in the following section suggest a more satisfactory explanation of the 

 above results. They will therefore be reconsidered after presentation 

 of the new findings. 



ganglia. Note the increase in size of ganglia of the treated embryo. FIG. 22. Spinal 

 ganglion of a control 11-day embryo. The two cell populations in the spinal gan- 

 glion ( MD and VL ) show the difference in their affinity for silver. FIG. 23. The 

 acetylcholinesterase activity in the V-L nerve cells of the spinal ganglia in 10- to 

 11-day chick embryos. ( Reproduced with the permission of M. A. Gerebtzoff from 

 Fig. 32 in Cholinesterases, Pergamon Press.) FIG. 24. Spinal ganglion contributing 

 nerve fibers to a fragment of mouse sarcoma 180 (not apparent in the picture). 

 Same magnification as Fig. 22. Note the size increase of the MD population. The 

 VL population does not show in this section. FIG. 25. The massive invasion of an 

 implanted fragment of mouse sarcoma 180 by nerve fibers from adjacent sensory 

 and sympathetic ganglia. T, tumor; S, sensory ganglion; SY, sympathetic ganglion. 



