262 The Nature of Biological Diversity 



adjacent epithelial tissues and no longer shares their fate. From that 

 moment, two major processes set in, to transform the thin neural plate 

 in that complex structure known as the vertehrate nervous system: 

 The one is the gross shaping of the hrain and of the spinal cord which 

 results from intense proliferative activity and from massive cell move- 

 ments; the other is the elaboration of a multitude of nerve centers 

 and of the wiring which interconnects these centers in the general 

 framework of the central nervous system. Obviously no sharp line 

 can be drawn between these two processes, since they occur simul- 

 taneously and the first sets the stage for the second, but they can be 

 singled out and analyzed as two distinct aspects of the same develop- 

 mental process. 



The analysis of the factors involved in the shaping of the brain and 

 the determination of its regional differences has been a favorite object 

 of embryologists ever since Spemann and Mangold discovered the 

 organizing potency of the upper blastoporal lip in amphibians. The 

 molding of the brain and of the spinal cord texture in a myriad of cell 

 aggregates, the formation of fiber tracts, and the outgrowth of periph- 

 eral nerves are instead the object of analysis of the neurologist, who 

 explores the developing nerve centers as a clue to the understanding 

 of their ultimate structure and function. 



To the biologist who is preoccupied not with the function in the 

 nervous system, but with the more general problems of growth and 

 differentiation, the developing nervous system offers an unequaled 

 field of exploration, for there he finds cell populations differentiating 

 along divergent lines in restricted temporal and spatial dimensions. 



To the writer, who shares with the neurologist the interest in struc- 

 ture and function in the nervous system, and with the biologist the 

 curiosity in growth and differentiative processes, the intricacies of 

 the nerve structures represent a never-ending object of wonder and 

 exploration. In the following pages only a few of the manifold aspects 

 of this problem will be considered. The selection is guided by per- 

 sonal experience and interest in some specific developmental aspects 

 rather than by a logical sequential line. 



At first we shall consider the developmental mechanisms which 

 operate in the early segregation and differentiation of nerve centers. 

 Then we shall discuss the effects produced on sensory nerve cells by 

 changes inflicted to their peripheral fields of innervation. Finally we 

 shall report on the growth response of sensory and sympathetic nerve 

 cells to some recently discovered nerve growth factors. The similar- 

 ities and differences between the response of sensory cells to nerve 

 growth factors and to an increased field of innervation will offer an 



