554 



COMPARATIVE ANATOMY 



methods of fixation by no means proves that they are artifacts. The 

 evidence, however, that they are primary in origin is not convincing. 

 Moreover, the supporters of the hypothesis have failed to show that 

 plasmodesms become nervous. 



The Balfour H3rpothesis. The distinguished British embryologist, 

 Balfour, maintained that embryonic nerves are cellular and that nerves 

 are modified cell chains. He concluded that all nerve cells are ectodermal 

 in origin; that sensory nerves develop from chains of cells derived from 

 the neural crest, and motor nerves come from cells which emerge from 



SPINAL CORD 



SENSORY 

 ■^r.;:ir— GANGLION 

 CELLS 



SCLEROTOME 



MESENCHYME 



SOMATIC 



MOTOR 



NEUROBLASTS 



r-T-T^ r- NOTOCHORD 





Fig. 457. — A portion of a cross section of a chick embryo showing the effect of the 

 ordinary embryological method of preservation and staining (on the left) in contrast 

 with that of a special neurological method such as that of Cajal (on the right). It is 

 not surprising that the chain theory of the origin of nerve fibers long seemed supported 

 by the study of sections of embryos prepared by the earlier methods. (Redrawn after 

 B. Patten.) 



the brain or spinal cord. The connexions of nerve and end organ are 

 therefore secondary. 



The histogenesis of a nerve fiber, according to Balfour, involves first 

 the arrangement of spindle-shaped ectodermal cells in a chain extending 

 from the central nervous system to the end organ. Each spindle-shaped 

 cell becomes differentiated into an axial thread and a peripheral proto- 

 plasmic non-nervous sheath. The axial threads unite end to end to form 

 the neurite, while the enclosing nucleated protoplasm becomes the neuri- 

 lemma sheath which is thus formed of a series or chain of cells as in the 

 adult nerve fiber. In medullated nerve fibers, the fatty medullary sheath 

 arises later between the neurite and the neurilemma. 



The Balfour hypothesis leaves unsolved the question how the chain 

 of cells which forms a nerve fiber is directed in its development towards 



