THE NERVOUS SYSTEM 255 



There are one or two vestigial ganglia behind the thirty-eighth nerve, 

 evidently in process of disappearance at eight days. 



The early history of the spinal nerves is as follows: The axis 

 cylinder processes of the fibers begin to grow^ out from the neuro- 

 blasts about the third day (cf. p. 235). At this time the myo- 

 tomes are in almost immediate contact with the ganglia; thus 

 the fibers have to cross only a very short space before they enter 

 the myotome. The further growth is associated with the growth 

 and differentiation of the myotome between which and the 

 embryonic nerve there is a very intimate relation of such a sort 

 that the nerve follows the myotome and its derivatives in all 

 changes of position. Thus nerves do not need to grow long 

 distances to establish their connections, but these are formed 

 at a very early period. This accounts for the motor fibers; the 

 way in which the sensory fibers, that arise from the spinal ganglia, 

 reach their termination is not known. 



Sheath-cells and Cell-chain Hypothesis. No embryonic nerve 

 consists entirely of axones, but, from the start, each nerve trunk 

 contains numerous nuclei. The latter belong to cells which have 

 been given two radically different interpretations, corresponding 

 to two distinct theories concerning the neuraxone. 



(1) The first theory, knowm as the neurone theory, is the one 

 tacitly followed in the preceding description and may be stated 

 as follows: the nerve-cell, dendrites and axone, including the 

 terminal arborization, constitute a single cellular individual or 

 unit, differentiated from the neuroblast alone. The nuclei in 

 the embryonic nerves therefore belong to cells that are foreign 

 to the primary nerve. Their function is to form the various 

 sheaths of the nerves, viz., the sheaths of the individual axones 

 and the endo-, peri-, and epineurium. The sheath of Schwann 

 arises from such cells that envelop the individual fibers at suitable 

 distances and spread longitudinally until neighboring sheath cells 

 meet; each such place of meeting constitutes a node of Ranvier. 

 Until recently it has been universally believed that the sheath 

 cells arose from the mesenchvme; but recent observations on Am- 

 phibia and Selachia have shown that they arise from the ganglia 

 in these forms; their original source is therefore the ectoderm. It 

 is probable that they have the same origin in the chick, though this 

 has not been demonstrated by direct observation or experiment. 



(2) The second theory is known as the cell-chain hypothesis. 



