Growth and Differentiation in the Nervous System 263 



opportunity of discussing one of the most obscure and yet important 

 aspects of growth and differentiation in the nervous system — the con- 

 trol mechanism exerted hy extrinsic factors on these processes. 



I. Developmental 3teehanisms in the 

 Central Nervous System 



The following analysis is based on the study of the developing 

 nervous system of the chick embryo, which has been our favorite 

 object of investigation during the past years. A few observations per- 

 formed on the nervous system of a mammalian embryo (mouse) show 

 that the developmental processes in birds and mammals follow a simi- 

 lar pattern. 



a. First differentiatire phase 



The band of thickened ectoderm along the middorsal line of the 

 chick embryo has folded into a neural groove and the neural groove 

 has changed in turn into a neural tube, before the cylindrical-shaped 

 cells which form the tube show any sign of their essence. In the head 

 region, differential growth of the walls results in the shaping of the 

 tube in three vesicles. The further elaboration of these vesicles in the 

 main brain regions will not be discussed here. Rather we shall consider 

 the differentiative waves which sweep the neural tube along its rostro- 

 caudal axis and lay down the building blocks of brain and spinal cord 

 nerve centers. 



It may be worth mentioning that this first and the following differ- 

 entiative processes would perhaps have never come to the attention 

 of the neurologist, were it not for the discovery by Cajal, toward the 

 end of the past century, of the unusually high affinity of differen- 

 tiating nerve cells for silver salts. The silver technique introduced by 

 Cajal offers an invaluable tool to detect the early differentiated nerve 

 cells, which stand out in sharp relief for their dark brown color on 

 the pale background of the undifferentiated cells. 



The neural tube has barely formed, when a swarming of neuro- 

 blasts takes place from the ventromedial lining of the central canal 

 to the lateral aspect of the tube. In the head region, where this differ- 

 entiative process first starts, the slender migrating neuroblasts are 

 the forerunner of the diencephalic-mesencephalic main nuclear ag- 

 gregates and of the motor cephalic nuclei. This process has been 

 described in detail in previous publications ( 1, 2, 3 ) and will not be 

 considered here. In the spinal cord, the neuroblasts build up a con- 

 tinuous column in a ventrolateral position. The further differentiation 



