THE FINAL ADJUSTMENTS II3 



tensions set up in the developing embryo. Many 

 processes of development involve the movement of 

 cells from one place to another inside the embryo, 

 and we know that cells will often move along lines 

 of orientated particles. In this way the tensions may 

 lay down the paths followed by the streams of cells 

 and thus determine where they arrive. 



The Nervous System and Development 



The nervous system, which is so important for the 

 integration of the adult body, might be expected to 

 play an important part in enabHng the embryo to 

 develop harmoniously. Actually it has been found to 

 be less effective in this particular respect than one 

 might suppose, but many other interesting points 

 arise about it. Before the nervous system can integrate 

 the parts of the body, the nerves must grow out 

 from the neural tube and reach the organs which they 

 finally innervate. While the nerves are growing the 

 paths along which they move are fixed by the posi- 

 tions of the organs to which they will become 

 attached. If a limb-rudiment is cut out of its normal 

 situation and grafted a short distance away, the 

 nerves bend out of their usual course to follow it. 

 The developing leg seems to attract them. This 

 attraction can be exerted by most sorts of developing 

 tissue, and if a leg is cut off and a nose grafted 

 nearby, the leg nerve fibres cheerfully grow off 

 towards the nose. But the actual way in which the 

 nerve joins on to the leg muscles or the nose is 

 another matter, and is more rigidly fixed. 



