278 The Nature of Biological Diversity 



These last investigations gave definite support to the concept that 

 diffusible agents control cell movements. Since these movements (as 

 in the case of migrating nerve cells) occur simultaneously with other 

 differentiative processes, it is difficult to draw a line between the 

 effects exerted by these agents on cell movements and on other aspects 

 of cell differentiation. 



In the case of the migratory movements in the central nervous sys- 

 tem, we are under no illusion that the processes might lend themselves 

 to a precise experimental analysis. Even though the hypothesis that 

 the complex locomotion pattern of nerve cells might he directed by 

 some agent released in the matrix by other cells or by the matrix itself 

 is plausible, the demonstration would be extremely more difficult than 

 in other situations where cell movements occur in more suitable en- 

 vironmental conditions. This conclusion should not discourage further 

 attempts to explore the complex migratory patterns of nerve cells, 

 since this exploration will undoubtedly contribute to our knowledge 

 of the developing nervous system. The results of the experimental 

 analysis of the spinal cord by Shieh (see page 268) and results of 

 experiments now in progress on the brain stem show that the migra- 

 tory patterns can be altered by transposition or ablation of segments 

 of the neural tube. They give evidence of the flexibility of these 

 processes and indicate that environmental factors play a role in the 

 differentiation of the central nervous system. 



We shall devote the following section to the analysis of develop- 

 mental and growth processes of nerve cells which early in life migrate 

 out of the central nervous system and establish themselves in discrete 

 cell aggregates known as sensory and sympathetic ganglia. This posi- 

 tion is most favorable for exploring the cells under normal and 

 experimental conditions. The results to be reported below show that 

 these cells are highly receptive to agents present in the medium not 

 only during embryonic life but throughout all their life cycle. 



If. Hole of Peripheral Effectors and Receptors 

 in the Differentiation and Growth 

 of Nerve Centers 



When in 1935 R. G. Harrison presented in the Croonian Lecture 

 the results of his brilliant analysis of the developing nervous system 

 of the amphibian (24), the newly opened field of experimental neuro- 

 embryology seemed to offer unlimited possibility to explore the 

 nervous system and to uncover the mechanisms which operate in its 

 differentiation. 



