100 



NERVOUS SYSTEM 



III. Syncy tied nucleated network theory (0. Sclmltze). There is a primary nexus between central 

 and end organ in the form of a network of anastomosing cells, which becomes differentiated 



nto nerve-fibres. Apathy's conception of a plexus of ' nerve '-cells in which neurofibrillae are 

 deposited to join up ' ganglion '-cell and end-organ is closely akin to this. 



IV. Outgrowth with early union theory (Hertwigs). The end organ is brought very early 

 nto connexion with the central organ by a protoplasmic process which is differentiated into a 



nerve and pulled out as the organs draw apart. 



V. Primitive-nerve theory (Baer, Hensen, Sedgwick). There is a primary nexus between 

 central and end organ in the form of protoplasmic strands which become fibrillar, and are drawn 

 out into nerve-fibres as the organs draw apart. 



VI. Primitive nexus and outgrowth theory (Held). There is a primitive protoplasmic nexus 

 between central and end organ, along which neurofibrillse grow from the nerve-cell outwards. 



In the first stage of the nerve the path is 

 non-nuclear, being furnished by Szily's 

 inter-epithelial network (see p. 59) ; in the 

 second stage it is nucleated, being provided 

 by the cellular (mesenchymic) syncytium. 



The sheath of Schwann is variously inter- 

 preted. According to theories III. and IV., 

 the nuclei of the sheath are those of the cells 

 entering into the formation of the nerve- 

 fibre, while the remaining hypotheses refer 

 them to a secondary investment either of 

 mesenchyme, or of ectoderm cells derived 

 from the ganglion-crest, or from the neural 

 tube along the motor root. It has been 

 stated above that the fibrous stage of the 

 dorsal roots of the ganglia is preceded by 

 a cellular stage, and the same is true of 

 the peripheral branches of certain purely 

 sensory nerves (A. F. Dixon l and others). 

 When the fibrous stage is established 

 these cells envelop the nerve-fibres. It 

 would appear probable from this that 

 the sheath of Schwann has an ectodermic 

 origin in sensory nerves ; and that the same 

 is true of the motor nerve -fibres is pointed 

 to by Harrison's experiments in which 

 the motor nerves were found to develop 

 without a cellular sheath in tadpoles from 

 which the neural crest had been removed 

 by operation (the dorsal roots remaining of 

 course undeveloped). This derivation of 

 sheath-cells from the neural-crest ectod< 

 has suggested that the peripheral extens 

 of the nerves may be associated with 



FIG. 187. SECTION OF SPINAL CORD OF FOUR- 

 WEEKS HUMAN EMBRYO. (His.) 



The dorsal roots are continued within the 

 cord into a small longitudinal bundle which is 

 the rudiment of the dorsal white column. The 

 anterior roots are formed by the convergence of 

 the processes of the neuroblasts. The latter, along 

 with the elongated cells of the myelospongium, 

 compose the grey matter. The external layer of 

 the cord is traversed by radiating fibres which 

 are the outer ends of the spongioblasts. The 

 ventral commissure is beginning to appear. 



continuous proliferation of indifferent eel 



which may become ganglion -cells in the sympathetic, or sheath-cells, in the nerve-tru 

 (Kohn). On the other hand, the evidence is strong in some cases that the mesenchy 

 cells furnish a nutritive sheath to the developing nerve (figs. 133, 134), and it see 

 not impossible that the nuclei in and around the nerve-path may be derived from 

 sources. 



The outgrowth- theory of His has been very generally accepted by anatomisl 

 and physiologists as the embryological basis of the Neurone-theory, and it hs 

 obtained its chief support from the Golgi method of staining, as applied moi 

 especially by Kamon y Cajal. But, as will be obvious from the above stateinei 

 of the various views which are prevalent on this subject, His' theory is by m 

 means universally admitted. The work of Held seems to show that, while 



1 Trans. Roy. Dublin Soc. vi. (Series IT.), 1896. 





