Growth and Differentiation in the Nervous System 281 



tion or the implantation of an additional limb at the same stage pro- 

 vided the conditions for this investigation. He observed severe atrophv 

 of the ganglia deprived of their peripheral field and noticeable size 

 increase in the ganglia confronted with a larger than normal field of 

 innervation. His results were confirmed in subsequent investigations 

 (31, 32). In 1943, while performing this analysis, we observed two 

 distinct classes of cells in the sensory ganglia (31). The presence in 

 these ganglia of two populations differing from each other in cell size 

 as well as in their location in the ganglia was first described by Ham- 

 burger in his early investigation of normal embryos and embryos 

 deprived of one limb bud (29). We now observed that the two cell 

 populations differ strikingly from each other also in their affinity for 

 silver. The lateroventral population is the first to differentiate and is 

 formed of large nerve cells which react intensively to silver from 

 their first differentiation at 4 to 5 days to the end of the incubation 

 period. The mediodorsal population does not show silver affinity till 

 the end of the incubation period. The two cell groups, which we 

 designated in a previous publication (32) as V-L and M-D from their 

 position, differ therefore from each other in time pattern of differen- 

 tiation, cell size, position, and silver affinity (Fig. 22). Recently a 

 cytochemical difference between these two types of neurons was 

 shown by Gerebtzoff. who investigated the acetyl-cholinesterase ac- 

 tivity in different cells and fiber systems ( 33 I . Although the author 

 called attention only to the different content of the enzyme in the 

 fibers of the dorsal funiculi, his photographic documentation shows 

 a clear-cut difference in the amount of the enzyme present in the two 

 cell populations: The V-L but not the M-D cells show the presence 

 of the enzyme (Fig. 23). The structural and chemical differences be- 

 tween the two types of sensory nerve cells come in sharp relief when 

 the ganglia are confronted with changes in their field of innervation. 

 In 1944 (31), we observed that the extirpation of the limb bud at 3 

 days of incubation results in an almost immediate breakdown and 

 disappearance of most of the V-L cells, whereas the M-D cells persist 

 in atrophic condition till the end of the incubation period. In 1949 

 ( 32 ) , we described a differential response of these two cell popula- 

 tions following implantation of an additional limb bud in 3-day-old 

 embryos. Counts of the V-L neurons in ganglia confronted with an 

 enlarged field showed an increase of about 80 per cent of these neu- 

 rons over the control cell population, whereas no changes were de- 

 tected in the number of the M-D cells. Results to he presented in the 

 following section gave evidence of a differential and opposite response 

 of the two types of neurons when confronted with neoplastic tissue. 



