THE NERVOUS SYSTEM 



555 



the end-organ. But this difficulty confronts any hypothesis which 

 assumes a secondary connexion between nerve and end organ. The 

 Balfour hypothesis had respectable standing only as long as special 

 methods of staining embryonic nerves had not been invented, or were 

 unused. When methods of staining which were specific for embryonic 

 nerves were finally employed by Golgi, Cajal, Bielschowsky, Ranson, 

 and others, the foundations of the Balfour hypothesis were gradually 

 undermined. It was found that embryonic nerves are fibrillar from the 



EPIDERMIS 



MYOTOME- }V^ 



SOMATIC 

 MOnOR NERVE 



SCLEROTOME 



NEURAL TUBE 



NEUROBLAST 

 CELLS 



NOTOCHORD 



Fig. 458. — A portion oi a bt-Liuai o. a 7 mm. elasmobranch embryo in the trunk 

 showing an early stage m the de\ elopmcnt of a somatic motor nerve, Bielschowsky 

 preparation. The nervous connexion is secondary and not formed by the enlargement 

 of a plasmodesm. The nervous character of the connexion is attested by its affinity 

 for special nerve stains. That the nerve anlage is formed by processes of neuroblast 

 cells in the spinal cord is further evidenced by the fact that processes of medullary 

 neuroblasts may be traced into the nerve anlage. Moreover, deeply staining neuro- 

 blasts are differentiated in the wall before the nerve anlage appears, as would be expected 

 if they form the nerve. 



time of their first appearance, and that sheath cells are a secondary 

 addition. Finally, the experiments of Harrison were decisive against this 

 hypothesis and in favor of the process theory of Kupffer. 



The Kupffer Theory. The German anatomist, Kupffer, was the 

 first to suggest that a nerve fiber becomes connected with its end- 

 organ by means of protoplasmic outflow from a neuroblast cell either in 

 the spinal cord or in the neural crest cells. Hence his theory became 

 known as the process theory of nerve development. Kupffer's theory 

 of neurogenesis may be better understood by taking as an example the 

 development of a somatic motor nerve fiber in a vertebrate embryo. 

 The neurite of such a fiber is formed by the protoplasmic outflow from a 

 neuroblast cell located in the ventro-lateral waU of the neural tube. 

 Neural tube and myotome are primarily unconnected. The neuroblast 

 cell before it forms a process is a spindle-shaped epithelial cell in the wall 



