. Comparative anatomy. Anatomy, Comparative. 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 fr
. Comparative anatomy. Anatomy, Comparative. 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 t
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