Growth and Differentiation in the Nervous System 279 



Three aspects of this analysis impressed particularly the reader of 

 the Croonian Lecture and encouraged further experimentation along 

 this line: (1) the extraordinary tolerance of the embryo for any kind 

 of surgical performance, ranging from restricted ahlations to destruc- 

 tion of most of its nervous structures; (2) the flexibility of the de- 

 veloping nervous system, which readily adapts itself to any new situ- 

 ation and even accepts foreign tissues as peripheral receptors and 

 effectors; and (3) the rapidity of the response of developing nerve 

 centers to changes inflicted to their fields of innervation. The situation 

 in the early thirties seemed to he very similar to the situation which 

 prevailed a few years earlier in the field of experimental embryology, 

 when the discovery of embryonic induction had transformed the 

 static field of descriptive embryology into one of the most dynamic 

 and promising areas of biology. In the years which followed the publi- 

 cation of the Croonian Lecture, all possible avenues of this new field 

 were explored. First the amphibian larvae and then the chick em- 

 bryos submitted obediently to the glass or steel needle, which per- 

 formed all kinds of ablation and transplantation experiments. Many 

 times during the past years the results and perspective in this field 

 have been considered (25, 26, 27) . We shall examine here only one of 

 the most discussed problems of neuroembryology — the mechanism of 

 control exerted by peripheral effectors and receptors on their asso- 

 ciated nerve centers. Since the first experiments were performed by 

 Harrison. Detwiler, and their students (24, 28) a good deal lias been 

 learned about the effects elicited by decreasing or increasing the 

 peripheral field of innervation, while practically nothing has been 

 learned about the regulatory mechanisms which operate under normal 

 and experimental conditions. Yet it is the understanding of these 

 mechanisms which is important if we wish to gain information on 

 growth and developmental processes in the nervous system. 



In the past the attention has been mainly focused on the differentia- 

 tion of spinal ganglia. As a result we know today more about the 

 differentiative pattern of sensory nerve cells than about any other 

 nerve cell. Most of the investigation centered on the chick embryo, 

 which provides excellent material for normal and experimental anal- 

 ysis. We shall consider in the following section the results of this 

 analysis, which recently received new impulse from the discovery of 

 specific nerve growth factors well identified in their chemical and 

 biological aspects. A closely related cell, the sympathetic nerve cell, 

 will also be considered since its response to growth factors, which is 

 even more impressive than the response of the sensory cell, sheds 

 light on other facets of the problem under consideration. 



