Forebrain 



Midbrain 



Hindbrain 



Ear primordi 



Fig. 33. The development 

 of the brain and associated 

 sense organs (after Wad- 

 dington ). 



organs and tissues. Cells that had previously been crowded out of the 

 neural folds ( the neural crest cells in Fig. 31 ) migrate down toward the 

 cord and aggregate into cell masses called ganglia. These cells become 

 sensory neurons and send dendrites to connect up with the dorsal part 

 of the spinal cord and axons into surrounding tissues. In this manner the 

 complete sensory-motor network is constructed. 



The growth of the nerve fibers and their penetration into peripheral 

 tissues is a fascinating subject. The process has been studied: (a) in the 

 normally developing embryo, ( b ) by cutting functional nerves in order to 

 follow the regeneration of a fiber from the old stump, and ( c ) by separat- 

 ing embryonic nerve cells and growing them in an artificial medium ( usu- 

 ally on a blood clot surrounded by blood serum and other constituents 

 found by experience to be delectable to a nerve cell). The presumptive 

 nerve cell body appears to send out an amoeboid process; the nucleus 

 remains within the cell body. Protoplasm flows into the process causing 

 it to move ahead while still connected to the cell body by a thin proto- 

 plasmic strand. This is the fiber. Special cells called Schwann cells then 

 flatten out and wrap themselves around the fiber in much the same way 

 that insulation is wrapped around an electrical conductor. 



The first fibers to penetrate into surrounding tissues are called 

 pioneer fibers and these are followed by others which apply themselves 

 to the pioneer and form a cable. Many problems involving the mechanism 

 of nerve growth remain unsolved. For example, how does a fiber know 

 to what specific organ or tissue it must go, and how does it know when it 

 has gotten there? How do the follower fibers duplicate the precise route 

 of the pioneer fiber? Why do only fibers of similar function collect into 

 cables? What is the control system which ensures that certain tissues will 

 receive many sensory and motor fibers while others will be innervated 

 only sparsely? 



64 



