290 The Nature of Biological Diversity 



mouse salivary glands and in their homologue, the snake venom 

 glands; it is present in lesser amount in the salivary glands of other 

 rodents, which in turn seem to possess it in higher amount in the 

 kidney (unpublished ohservations) . It is present in embryonic tissues 

 of chick embryo (45) and in granuloma tissue experimentally pro- 

 duced in different vertebrates (44) . It is also detectable in the sensory 

 and sympathetic nerve cells of embryonic birds and in the sympathetic 

 cells of a variety of mammals, man included. A preliminary explora- 

 tion indicates its presence in human serum. 



The functional significance of this agent in the growth and develop- 

 mental processes of sympathetic nerve cells can hardly be questioned. 

 We have evidence that in the mouse the sympathetic cells remain 

 receptive to the NGF throughout life and it is conceivable (but still 

 not proved ) that this applies also to the same cells in other mammals 

 (Fig. 34) . Additional support in favor of the hypothesis that the NGF 

 might play an essential role in the growth and differentiation of the 

 receptive nerve cells came from experiments reported in detail else- 

 where (41, 46), which showed that the injection of this antiserum 

 destroys selectively the sympathetic ganglia in newborn mammals 

 (Fig. 29). 



A re-evaluation of the role of extrinsic 

 factors as controlling agents 

 of growth and differentiation 

 of nerve cells 



Before attempting an evaluation of the results presented in the 

 preceding pages, it may be of interest to consider how past interpreta- 

 tions were abandoned as new facts were discovered and as we gained 

 a more precise picture of the phenomenon. We believe that the 

 present information is still at best only partial and fragmentary; this 

 belief and past experience suggest offering a working hypothesis 

 rather than a conclusion in the following pages. 



It will be recalled that at first the growth effects elicited by mouse 

 sarcomas on the sensory ganglia of the chick embryo were compared 

 to the effects elicited by implantation of an additional limb in the 

 embryo (34) . At that time, the similarities rather than the differences 

 between the two effects were stressed. A reinvestigation and a more 

 close inspection of the phenomenon resulted in a radical change of 

 position. When it was discovered that the tumor elicits a generalized 

 growth effect on the sensory and sympathetic ganglia of the embryo, 

 the differences rather than the similarities between the effects of the 



